diff --git a/Foxnouns.Frontend/app/components/ErrorAlert.tsx b/Foxnouns.Frontend/app/components/ErrorAlert.tsx
index e30c516..cedb69a 100644
--- a/Foxnouns.Frontend/app/components/ErrorAlert.tsx
+++ b/Foxnouns.Frontend/app/components/ErrorAlert.tsx
@@ -1,7 +1,13 @@
 import { TFunction } from "i18next";
 import Alert from "react-bootstrap/Alert";
-import { useTranslation } from "react-i18next";
-import { ApiError, ErrorCode } from "~/lib/api/error";
+import { Trans, useTranslation } from "react-i18next";
+import {
+	ApiError,
+	ErrorCode,
+	ValidationError,
+	validationErrorType,
+	ValidationErrorType,
+} from "~/lib/api/error";
 
 export default function ErrorAlert({ error }: { error: ApiError }) {
 	const { t } = useTranslation();
@@ -10,10 +16,112 @@ export default function ErrorAlert({ error }: { error: ApiError }) {
 		<Alert variant="danger">
 			<Alert.Heading as="h4">{t("error.heading")}</Alert.Heading>
 			{errorCodeDesc(t, error.code)}
+			{error.errors && (
+				<ul>
+					{error.errors.map((e, i) => (
+						<ValidationErrors key={i} errorKey={e.key} errors={e.errors} />
+					))}
+				</ul>
+			)}
 		</Alert>
 	);
 }
 
+function ValidationErrors({ errorKey, errors }: { errorKey: string; errors: ValidationError[] }) {
+	return (
+		<li>
+			<strong>
+				<code>{errorKey}</code>
+			</strong>
+			:
+			<ul>
+				{errors.map((e, i) => (
+					<li key={i}>
+						<ValidationErrorEntry error={e} />
+					</li>
+				))}
+			</ul>
+		</li>
+	);
+}
+
+function ValidationErrorEntry({ error }: { error: ValidationError }) {
+	const { t } = useTranslation();
+
+	const {
+		min_length: minLength,
+		max_length: maxLength,
+		actual_length: actualLength,
+		message: reason,
+		actual_value: actualValue,
+		allowed_values: allowedValues,
+	} = error;
+
+	switch (validationErrorType(error)) {
+		case ValidationErrorType.LengthError:
+			if (error.actual_length! > error.max_length!) {
+				return (
+					<Trans
+						t={t}
+						i18nKey={"error.validation.too-long"}
+						values={{ maxLength: error.max_length!, actualLength: error.actual_length! }}
+					>
+						Value is too long, maximum length is {{ maxLength }}, current length is{" "}
+						{{ actualLength }}.
+					</Trans>
+				);
+			}
+
+			if (error.actual_length! < error.min_length!) {
+				return (
+					<Trans
+						t={t}
+						i18nKey={"error.validation.too-short"}
+						values={{ minLength: error.min_length!, actualLength: error.actual_length! }}
+					>
+						Value is too short, minimum length is {{ minLength }}, current length is{" "}
+						{{ actualLength }}.
+					</Trans>
+				);
+			}
+
+			break;
+
+		case ValidationErrorType.DisallowedValueError:
+			return (
+				<Trans
+					t={t}
+					i18nKey={"error.validation.disallowed-value"}
+					values={{
+						actualValue: error.actual_value!.toString(),
+						allowedValues: error.allowed_values!.map((v) => v.toString()).join(", "),
+					}}
+				>
+					{/* @ts-expect-error i18next handles interpolation */}
+					The value <code>{{ actualValue }}</code> is not allowed here. Allowed values are:{" "}
+					{/* @ts-expect-error i18next handles interpolation */}
+					<code>{{ allowedValues }}</code>
+				</Trans>
+			);
+
+		default:
+			if (error.actual_value) {
+				return (
+					<Trans
+						t={t}
+						i18nKey={"error.validation.generic"}
+						values={{ actualValue: error.actual_value!.toString(), reason: error.message }}
+					>
+						{/* @ts-expect-error i18next handles interpolation */}
+						The value <code>{{ actualValue }}</code> is not allowed here. Reason: {{ reason }}
+					</Trans>
+				);
+			}
+
+			return <>{t("error.validation.generic-no-value", { reason: error.message })}</>;
+	}
+}
+
 export const errorCodeDesc = (t: TFunction, code: ErrorCode) => {
 	switch (code) {
 		case ErrorCode.AuthenticationError:
diff --git a/Foxnouns.Frontend/app/lib/api/error.ts b/Foxnouns.Frontend/app/lib/api/error.ts
index 02e871c..0a3d9b9 100644
--- a/Foxnouns.Frontend/app/lib/api/error.ts
+++ b/Foxnouns.Frontend/app/lib/api/error.ts
@@ -3,7 +3,7 @@ export type ApiError = {
 	status: number;
 	message: string;
 	code: ErrorCode;
-	errors?: ValidationError[];
+	errors?: Array<{ key: string; errors: ValidationError[] }>;
 };
 
 export enum ErrorCode {
@@ -26,3 +26,31 @@ export type ValidationError = {
 	allowed_values?: any[];
 	actual_value?: any;
 };
+
+/**
+ * Returns the first error for the value `key` in `error`.
+ * @param error The error object to traverse.
+ * @param key The JSON key to find.
+ */
+export const firstErrorFor = (error: ApiError, key: string): ValidationError | undefined => {
+	if (!error.errors) return undefined;
+	const field = error.errors.find((e) => e.key == key);
+	if (!field?.errors) return undefined;
+	return field.errors.length != 0 ? field.errors[0] : undefined;
+};
+
+export enum ValidationErrorType {
+	LengthError = 0,
+	DisallowedValueError = 1,
+	GenericValidationError = 2,
+}
+
+export const validationErrorType = (error: ValidationError) => {
+	if (error.min_length && error.max_length && error.actual_length) {
+		return ValidationErrorType.LengthError;
+	}
+	if (error.allowed_values && error.actual_value) {
+		return ValidationErrorType.DisallowedValueError;
+	}
+	return ValidationErrorType.GenericValidationError;
+};
diff --git a/Foxnouns.Frontend/app/routes/auth.callback.discord/route.tsx b/Foxnouns.Frontend/app/routes/auth.callback.discord/route.tsx
index f946ee0..31d4d64 100644
--- a/Foxnouns.Frontend/app/routes/auth.callback.discord/route.tsx
+++ b/Foxnouns.Frontend/app/routes/auth.callback.discord/route.tsx
@@ -1,11 +1,23 @@
-import { json, LoaderFunctionArgs } from "@remix-run/node";
-import { type ApiError, ErrorCode } from "~/lib/api/error";
+import { ActionFunctionArgs, json, redirect, LoaderFunctionArgs } from "@remix-run/node";
+import { type ApiError, ErrorCode, firstErrorFor } from "~/lib/api/error";
 import serverRequest, { writeCookie } from "~/lib/request.server";
-import { CallbackResponse } from "~/lib/api/auth";
-import { Form as RemixForm, Link, useLoaderData } from "@remix-run/react";
+import { AuthResponse, CallbackResponse } from "~/lib/api/auth";
+import {
+	Form as RemixForm,
+	Link,
+	useActionData,
+	useLoaderData,
+	ShouldRevalidateFunction,
+} from "@remix-run/react";
 import { Trans, useTranslation } from "react-i18next";
 import Form from "react-bootstrap/Form";
 import Button from "react-bootstrap/Button";
+import ErrorAlert from "~/components/ErrorAlert";
+import Alert from "react-bootstrap/Alert";
+
+export const shouldRevalidate: ShouldRevalidateFunction = ({ actionResult }) => {
+	return !actionResult;
+};
 
 export const loader = async ({ request }: LoaderFunctionArgs) => {
 	const url = new URL(request.url);
@@ -17,7 +29,7 @@ export const loader = async ({ request }: LoaderFunctionArgs) => {
 		throw { status: 400, code: ErrorCode.BadRequest, message: "Missing code or state" } as ApiError;
 
 	const resp = await serverRequest<CallbackResponse>("POST", "/auth/discord/callback", {
-		body: { code, state }
+		body: { code, state },
 	});
 
 	if (resp.has_account) {
@@ -25,9 +37,9 @@ export const loader = async ({ request }: LoaderFunctionArgs) => {
 			{ hasAccount: true, user: resp.user!, ticket: null, remoteUser: null },
 			{
 				headers: {
-					"Set-Cookie": writeCookie("pronounscc-token", resp.token!)
-				}
-			}
+					"Set-Cookie": writeCookie("pronounscc-token", resp.token!),
+				},
+			},
 		);
 	}
 
@@ -35,26 +47,62 @@ export const loader = async ({ request }: LoaderFunctionArgs) => {
 		hasAccount: false,
 		user: null,
 		ticket: resp.ticket!,
-		remoteUser: resp.remote_username!
+		remoteUser: resp.remote_username!,
 	});
 };
 
-// TODO: action function
+export const action = async ({ request }: ActionFunctionArgs) => {
+	const data = await request.formData();
+	const username = data.get("username") as string | null;
+	const ticket = data.get("ticket") as string | null;
+
+	if (!username || !ticket)
+		return json({
+			error: {
+				status: 403,
+				code: ErrorCode.BadRequest,
+				message: "Invalid username or ticket",
+			} as ApiError,
+			user: null,
+		});
+
+	try {
+		const resp = await serverRequest<AuthResponse>("POST", "/auth/discord/register", {
+			body: { username, ticket },
+		});
+
+		return redirect("/auth/welcome", {
+			headers: {
+				"Set-Cookie": writeCookie("pronounscc-token", resp.token),
+			},
+			status: 303,
+		});
+	} catch (e) {
+		JSON.stringify(e);
+
+		return json({ error: e as ApiError });
+	}
+};
 
 export default function DiscordCallbackPage() {
 	const { t } = useTranslation();
 	const data = useLoaderData<typeof loader>();
+	const actionData = useActionData<typeof action>();
 
 	if (data.hasAccount) {
 		const username = data.user!.username;
-		
+
 		return (
 			<>
 				<h1>{t("log-in.callback.success")}</h1>
 				<p>
-					<Trans t={t} i18nKey={"log-in.callback.success-link"} values={{ username: data.user!.username }}>
+					<Trans
+						t={t}
+						i18nKey={"log-in.callback.success-link"}
+						values={{ username: data.user!.username }}
+					>
 						{/* @ts-expect-error react-i18next handles interpolation here */}
-						Welcome back, <Link to={`/@${data.user!.username}`}>@{{username}}</Link>!
+						Welcome back, <Link to={`/@${data.user!.username}`}>@{{ username }}</Link>!
 					</Trans>
 					<br />
 					{t("log-in.callback.redirect-hint")}
@@ -66,6 +114,7 @@ export default function DiscordCallbackPage() {
 	return (
 		<RemixForm method="POST">
 			<Form as="div">
+				{actionData?.error && <RegisterError error={actionData.error} />}
 				<Form.Group className="mb-3" controlId="remote-username">
 					<Form.Label>{t("log-in.callback.remote-username.discord")}</Form.Label>
 					<Form.Control type="text" readOnly={true} value={data.remoteUser!} />
@@ -82,3 +131,34 @@ export default function DiscordCallbackPage() {
 		</RemixForm>
 	);
 }
+
+function RegisterError({ error }: { error: ApiError }) {
+	const { t } = useTranslation();
+
+	// TODO: maybe turn these messages into their own error codes?
+	const ticketMessage = firstErrorFor(error, "ticket")?.message;
+	const usernameMessage = firstErrorFor(error, "username")?.message;
+
+	if (ticketMessage === "Invalid ticket") {
+		return (
+			<Alert variant="danger">
+				<Alert.Heading as="h4">{t("error.heading")}</Alert.Heading>
+				<Trans t={t} i18nKey={"log-in.callback.invalid-ticket"}>
+					Invalid ticket (it might have been too long since you logged in with Discord), please{" "}
+					<Link to="/auth/log-in">try again</Link>.
+				</Trans>
+			</Alert>
+		);
+	}
+
+	if (usernameMessage === "Username is already taken") {
+		return (
+			<Alert variant="danger">
+				<Alert.Heading as="h4">{t("log-in.callback.invalid-username")}</Alert.Heading>
+				{t("log-in.callback.username-taken")}
+			</Alert>
+		);
+	}
+
+	return <ErrorAlert error={error} />;
+}
diff --git a/Foxnouns.Frontend/public/locales/en.json b/Foxnouns.Frontend/public/locales/en.json
index 83455f7..f2dfbf2 100644
--- a/Foxnouns.Frontend/public/locales/en.json
+++ b/Foxnouns.Frontend/public/locales/en.json
@@ -1,52 +1,62 @@
 {
-	"error": {
-		"heading": "An error occurred",
-		"errors": {
-			"authentication-error": "There was an error validating your credentials.",
-			"authentication-required": "You need to log in.",
-			"bad-request": "Server rejected your input, please check anything for errors.",
-			"forbidden": "You are not allowed to perform that action.",
-			"generic-error": "An unknown error occurred.",
-			"internal-server-error": "Server experienced an internal error, please try again later.",
-			"member-not-found": "Member not found, please check your spelling and try again.",
-			"user-not-found": "User not found, please check your spelling and try again."
-		},
-		"title": "Error"
-	},
-	"navbar": {
-		"view-profile": "View profile",
-		"settings": "Settings",
-		"log-out": "Log out",
-		"log-in": "Log in or sign up",
-		"theme": "Theme",
-		"theme-auto": "Automatic",
-		"theme-dark": "Dark",
-		"theme-light": "Light"
-	},
-	"log-in": {
-		"callback": {
-			"success": "Successfully logged in!",
-			"success-link": "Welcome back, <1>@{{username}}</1>!",
-			"redirect-hint": "If you're not redirected to your profile in a few seconds, press the link above.",
-			"remote-username": {
-				"discord": "Your discord username"
-			},
-			"username": "Username",
-			"sign-up-button": "Sign up"
-		},
-		"title": "Log in",
-		"form-title": "Log in with email",
-		"email": "Email address",
-		"password": "Password",
-		"log-in-button": "Log in",
-		"register-with-email": "Register with email",
-		"3rd-party": {
-			"title": "Log in with another service",
-			"desc": "If you prefer, you can also log in with one of these services:",
-			"discord": "Log in with Discord",
-			"google": "Log in with Google",
-			"tumblr": "Log in with Tumblr"
-		},
-		"invalid-credentials": "Invalid email address or password, please check your spelling and try again."
-	}
+  "error": {
+    "heading": "An error occurred",
+    "validation": {
+      "too-long": "Value is too long, maximum length is {{maxLength}}, current length is {{actualLength}}.",
+      "too-short": "Value is too short, minimum length is {{minLength}}, current length is {{actualLength}}.",
+      "disallowed-value": "The value <1>{{actualValue}}</1> is not allowed here. Allowed values are: <4>{{allowedValues}}</4>",
+      "generic": "The value <1>{{actualValue}}</1> is not allowed here. Reason: {{reason}}",
+      "generic-no-value": "The value you entered is not allowed here. Reason: {{reason}}"
+    },
+    "errors": {
+      "authentication-error": "There was an error validating your credentials.",
+      "authentication-required": "You need to log in.",
+      "bad-request": "Server rejected your input, please check anything for errors.",
+      "forbidden": "You are not allowed to perform that action.",
+      "generic-error": "An unknown error occurred.",
+      "internal-server-error": "Server experienced an internal error, please try again later.",
+      "member-not-found": "Member not found, please check your spelling and try again.",
+      "user-not-found": "User not found, please check your spelling and try again."
+    },
+    "title": "Error"
+  },
+  "navbar": {
+    "view-profile": "View profile",
+    "settings": "Settings",
+    "log-out": "Log out",
+    "log-in": "Log in or sign up",
+    "theme": "Theme",
+    "theme-auto": "Automatic",
+    "theme-dark": "Dark",
+    "theme-light": "Light"
+  },
+  "log-in": {
+    "callback": {
+      "success": "Successfully logged in!",
+      "success-link": "Welcome back, <1>@{{username}}</1>!",
+      "redirect-hint": "If you're not redirected to your profile in a few seconds, press the link above.",
+      "remote-username": {
+        "discord": "Your discord username"
+      },
+      "username": "Username",
+      "sign-up-button": "Sign up",
+      "invalid-ticket": "Invalid ticket (it might have been too long since you logged in with Discord), please <2>try again</2>.",
+      "invalid-username": "Invalid username",
+      "username-taken": "That username is already taken, please try something else."
+    },
+    "title": "Log in",
+    "form-title": "Log in with email",
+    "email": "Email address",
+    "password": "Password",
+    "log-in-button": "Log in",
+    "register-with-email": "Register with email",
+    "3rd-party": {
+      "title": "Log in with another service",
+      "desc": "If you prefer, you can also log in with one of these services:",
+      "discord": "Log in with Discord",
+      "google": "Log in with Google",
+      "tumblr": "Log in with Tumblr"
+    },
+    "invalid-credentials": "Invalid email address or password, please check your spelling and try again."
+  }
 }