Assassins are dangerous criminals hired to murder others. Assassins are lethal and have a mean attitude. The assassin is truly an elite human warrior, but luckily they are rare.
Assassins have lots of hit points and are the toughest human entity in Thief: The Dark Project and Thief Gold, taking third place behind Apparitions and Bugbeasts. On the other hand they have a high fleeing point and it takes relatively the same amount of effort to drive them away as it would take with a Hammerite Guard or a Hammerite Priest.
In offensive aspect the sword assassin does normal damage and has normal attack speed. The sword assassin does however, have a fast moving speed, similar to a Haunt. But the bow assassin has the devastating and extremely rare ability to shoot Fire Arrows (like the Sir Cullen's Keep archer guard in Thief 2). He won't always shoot a Fire Arrow, but he will alternate between a Broadhead Arrow and a Fire attack, in a seemingly random manner. A direct hit from a Fire Arrow is fatal on Expert difficulty.