I think this is the right rule

src/App.tsx

@@ -38,6 +38,7 @@ function App() {
   useEffect(() => {
     document.body.className = dark ? "dark" : "";
     setTimeout(() => {
+      // Avoid transition on page load
       document.body.style.transition = "0.3s background-color ease-out";
     }, 1);
   }, [dark]);
@@ -58,14 +59,10 @@ function App() {
     <div className="App-container">
       <h1>
         <span
-          style={
-            difficulty > 0
-              ? {
-                  color: "#e66",
-                  textShadow: difficulty > 1 ? "0px 0px 5px #e66" : "none",
-                }
-              : {}
-          }
+          style={{
+            color: difficulty > 0 ? "#e66" : "inherit",
+            fontStyle: difficulty > 1 ? "italic" : "inherit",
+          }}
         >
           hell
         </span>
@@ -136,9 +133,9 @@ function App() {
               >
                 {
                   [
-                    `No restrictions on guesses.`,
+                    `Guesses must be valid dictionary words.`,
                     `Wordle's "Hard Mode". Green letters must stay fixed, and yellow letters must be reused.`,
-                    `An even stricter Hard Mode. Yellow letters must move away from where they were clued.`,
+                    `An even stricter Hard Mode. Yellow letters must move away from where they were clued, and gray clues must be obeyed.`,
                   ][difficulty]
                 }
               </div>

src/clue.ts

@@ -1,4 +1,4 @@
-import { Difficulty, ordinal } from "./util";
+import { Difficulty, englishNumbers, ordinal } from "./util";
 
 export enum Clue {
   Absent,
@@ -71,9 +71,17 @@ export function violation(
     const upper = letter.toUpperCase();
     const nth = ordinal(i + 1);
     if (clue === Clue.Absent) {
-      // if (difficulty === Difficulty.UltraHard && guess.includes(letter)) {
-      //   return "Guess can't contain " + upper;
-      // }
+      if (difficulty === Difficulty.UltraHard) {
+        const max = clues.filter(
+          (c) => c.letter === letter && c.clue !== Clue.Absent
+        ).length;
+        const count = guess.split(letter).length - 1;
+        if (count > max) {
+          const amount = max ? `more than ${englishNumbers[max]} ` : "";
+          const s = max > 1 ? "s" : "";
+          return `Guess can't contain ${amount}${upper}${s}`;
+        }
+      }
     } else if (clue === Clue.Correct) {
       if (guess[i] !== letter) {
         return nth + " letter must be " + upper;

src/util.ts

@@ -59,3 +59,6 @@ export function speak(
 export function ordinal(n: number): string {
   return n + ([, "st", "nd", "rd"][(n % 100 >> 3) ^ 1 && n % 10] || "th");
 }
+
+export const englishNumbers =
+  "zero one two three four five six seven eight nine ten eleven".split(" ");