If
you cast
Mephistopheles, your
Avatar is
banished. Mephistopheles is now your Avatar. He retains
Unique rarity, Demon type, and
Air element, but he's no longer a
minion and only an Avatar. Your current and maximum
life remain the same. Notably, you won't be able to
play sites normally because Mephistopheles doesn't have that
ability. You will still be able to cast spells because Avatars are spellcasters as one of their
characteristics. If you manage to
summon Mephistopheles without casting him, then he will not replace your avatar, but his second ability will still function.