"Intelligent" enemies.

[EssenceofEvil]

What exactly does that apply to for the assassin enhancement?

[FranOhmsford]

What exactly does that apply to for the assassin enhancement?

Most likely the same as for everyone else.

None-Intelligent Enemies = Zombies, Skeletons, Golems, Oozes etc.

Other Undead are of varying Intelligence so there's a good chance that you can Assassinate Vampires, Wraiths etc.

[FranOhmsford]

Oh and plenty of Zombies and Skeletons are actually Intelligent in DDO - Black Loch Zombies most certainly are!

Skeleton Knights should be!
Skeleton Mages pretty much have to be!

[foolz]

It works on all kinds of stuff, skeletons, woads, elementals pretty much everything i've used it on so far.