Xolatoyac, the Smiling Flood (LCC-008) - The Lost Caverns of Ixalan Commander Foil

Whenever Xolatoyac enters or attacks, put a flood counter on target land. That land is an Island in addition to its other types for as long as it has a flood counter on it.
At the beginning of your end step, untap each permanent you control with a counter on it.