The rogue has a lot going on. In my opinion it is too advanced to really fit in an other wise vanilla game of Hero Quest.
But you asked if he was too OP.
The auto-hit-only-one-defense-die-daggers are very powerful, but they are basically one-use spells you even have to pay for. They seem balanced enough. Maybe even slightly underpowered?!?
The weapon restrictions are also a negative aspect. Certainly NOT OP.
Then we finally have the sneak attacks. Well, personally I don't like facing rules. Suddenly everyone spends extra time finding perfect spots.
The alternative is just gaining +3 dice on the FIRST attack during a battle. Need some clarification on what counts as a battle!
We assume the rogue quickly acquires the Longsword. 3 dice + diagonally.
That means a 6 dice attack once per battle. That's pretty powerful. Depending on when and if you can reset the ability it may or may not be OP.
Judgement:
All in all I don't think the Rogue is too OP. Strong attacks and spells, but paying gold and limited equipment. It all adds up.
But with
Trap disarming, special daggers, sneak attack, equipment restrictions, moving through monsters... This character is way too messy for my taste.
I'd pick a trait or two to give it rogue flavor.
If Carl Forhan had designed the Dwarf he would have Trap disarming, +1 dice against goblins, mine through fallen rocks, -2 to movement and a treasure hunter ability.
Keep it simple
