ToME: the Tales of Maj'Eyal

Due to the fact that stealth/invisibility is moderately broken, here is my idea for them

Stealth:
Make it so that monsters do not notice you (unless they see you, chances that they see you are increased if you are in an open square (no dungeon features adjacent to you (including walls)). Chance is based on Dex and cunning of the one in stealth, and the cunning of the one trying to find. If they notice you stealth is broken, since being noticed involves seeing, not hearing, at level 5, there is a 20% chance if you are noticed to not break stealth, but monster will move toward you.

Invisibility:

Make it so that you are invisible. Chance that monsters do not notice anything is based on your dex and cunning, and their cunning. If they notice you, they will move to the sound, but not target you unless they bump into you. They will not attack unless they bump into you, and then they will attack the place where they found you, unless they were able to pinpoint you in another location.

with both of them: Monsters do not attack you or move (unless they are patrolling) until they notice you, see above for what happens. <-- main difference

and there should be a caveat if you have both stealth and invisibility, making you practically undetectable unless they have a means of seeing through your invisibility. but that may make it too easy for shadowblades

Yeah...

Actually, you could make it that stealth enables you to walk "in" walls next to "floor" ie, you are flat against the wall. I think it'll be a nightmare to program, but... So in this example you can go to all spots except the x's... You break stealth if you move off a wall and a monster is in sight. You can only enter stealth while no monsters is around, and then you have to get onto a wall before a monster is within LOS. Movement speed is halved (but improves with dex) while in stealth mode.

Probably not too practical...