Assassin is one of the classes in several role-playing games. Most assassins make it their goal to sneak around and kill things. Assassins tend to be good at searching for things and dodging others' attacks.
An assassin is somewhat similar to a rogue, but whereas the rogues of ToME are better at employing magic, thieving items, and installing traps, the assassins are better at being stealthy and fighting with weapons.
Stealth means that monsters are less likely to notice assassins walking or running around in the dungeon. This is good, because it easier to fight a single monster without waking up a nearby group, and it is easier to flee from monsters without having the other monsters on that level wake up and pursue the assassin as well.
See also Documentation/Classes/Assassin at the ToME wiki or using the in-game help.
In EverQuest II
In EverQuest 2, the assassin is a chooseable scout class.