Ithara, Mistress of the Abyss

Vigilance, reach

Creatures don't untap during their controllers' untap steps.

Whenever a creature enters, untap all creatures.

Tap X untapped Merfolk you control: Ithara,
Mistress of the Abyss gets +X/+X until end of turn,
where X is the number of Merfolk tapped this way.