Target friendly champion gains all of the base and upgraded abilities of the closest non-Avatar/Boss/Elite champion within 1 space owned by an opposing player. Basic attacks and abilities that change a champion's form are not gained.
Note that only base and upgraded abilities are stolen (those which appear in the rune manager). This is a source of controversy due to being to combine abilities which are very strong together (such as Flurry and Cleave).
You can only copy your opponent's champions with this (even if they are possessed)-- the champion needs to be owned by a different player.