I'm not so sure about that. It depends exactly what it meant by "until the End Phase". The way I read it, Phantom of Chaos stops copying the monster during the non-existent gap between Main Phase 2 and End Phase. Therefore it's not copying Horus LV6 at the time it would level itself up (the End Phase), and thus it's impossible.Phantom of Chaos
Effect Monster (Fiend / DARK / 4 Stars / ATK 0 / DEF 0)
Any Battle Damage this monster inflicts to your opponent becomes 0. Once per turn, you can remove from play 1 Effect Monster in your Graveyard. Until the End Phase, this card's name is treated as that monster's name, and it gains that monster's effect(s) and ATK.
good enough for me Mini-Simon, thx.