Zelvaus, Underlord Phantom

When Zelvaus, Underlord Phantom enters the battlefield, return any number of creature
cards with total mana value X or less from your graveyard to the battlefield.

{4}{u/b}: Target creature gets an undead counter. (Creatures can only have one undead counter. Creatures with undead counters can be played from the graveyard by paying {2}{b}.)