Xola, The Ashes of Dusk

At the beginning of your end step, each player
loses life equal to Xola's power.
Sacrifice a Phoenix: put a +1/+1 counter on
Xola.
{t}Sacrifice two Phoenixes: Xola gains indestructible until the next end step.
"As the sun dipped below the horizon, She turned to
ash, and all life went with her."
— The Fables of Xola