The assassin's ability works when you "attack" a "creature" or "Character" in the new edition. Technically, since there are no "creatures" in the deck or on the board, this can only be used on Characters. Now that everything is keyworded for clarity, it should probably read "Enemy," the keyword on Adventure Cards, rather than "creature."
And since it refers to Combat, it probably means "non-spirit Enemy."
And since it says you "attack" rather than "when attacked by," it probably means "only face up, non-spirit Enemies on Adventure Cards when landed upon"
Ultimately, MUCH less powerful than some people are suggesting. Use against the werewolf/pitfiends is probably acceptable with the current wording, but is still a bit of a stretch.
That is what the rules read, but not how you're supposed to interpret them it seems. As here is the latest quote on the Assassin (from another thread):
"This rule is only a general rule to show new players how battles are resolved. The Assassin may assassinate any Enemy, whether or not he just drew it or landed on it."
"In summary, the Assassin may assassinate any creatures in battle, including the Sentinel, or the Pit Fiends or Werewolf in the Inner Region. Yes he has been beefed up for the revised edition. He may only assassinate another character when he attacks them, but not on the Crown of Command!"
from this thread: