As Mephisto's Soulstone enters the battlefield, put a corruption counter on it.

At the beginning of your upkeep, put a corruption counter on Mephisto's Soulstone.

{2}, Remove two corruption counters from Mephisto's Soulstone: Create a 2/2 black Demon creature token with flying.

{5}, Remove five corruption counters from Mephisto's Soulstone: Create Mephisto, a legendary 6/6 black and blue Demon creature token with flying, trample, and "Whenever this creature deals combat damage to a player, that player discards two cards."