Grigori Rasputin

How do I resolve this ability?

Rasputin must be in your cemetery to use this ability. Then, choose the Evil ally you will banish, which may be anywhere and not limited to your own sites. Since Banishing is the cost of the activated ability, resolve the whole activated ability before actually banishing the minion. (This is analogous to the 'Sacrifice' rules.) Rasputin gains the printed power and abilities of the ally. Resolve Genesis abilities and any 'enter the realm' abilities. Finally, the Banished ally is actually removed from the realm. (Place it Rasputin as a reminder of the current effect.)