This champion begins play Incorporeal (This Champion is immune to all damage except Magical. It cannot attack, but has flying and ignores all movement restrictions and penalties from terrain) for 2 turns and becomes Incorporeal for 2 turns at the start of each of your other turns.
Note that Ethereal is a strength as well as a weakness. While Incorporeal, champions can't attack or engage other champions, but they also can't be engaged or be take non-magical damage.
In order to destroy them, nearby opposing champions can save up AP to attack twice the turn the champion is not Incorporeal.