One of the unique powers of the Phantom Lantern is being able to transform into a Lantern of a different Corps. Before this, the closest skill set like this was from the Indigo tribe channeling the powers of the other Corps while maintaining their Indigo powers.
Not sure yet what the drawback is wielding the Phantom Ring — I can’t tell if the host, Frank Laminski, is already this unhinged before wearing the ring.
From – Green Lanterns Vol. 1 #11