Alith Anar:

Forced : When this legend enters play, you must burn 3 zones instead of 2 in order to win for the rest of the game. Action: When this legend attacks, turn target development you control faceup. If it is a [High Elf] unit with Ambush, trigger its ability as if it just ambushed and declare it as an attacker. Otherwise, sacrifice it.

Question:

Is the unit I turn faceup able to attack from any zone, or must it be in my battlefield in order to not create an illegal state?

Also, must you pay Ambush cost to do so? I would think not.

No ambush cost, can attack from any zone.