Hebdomas, Sculptor of Eras

Wall of Stone — Whenever you cast an Instant or
a spell with Flash, create a 0/X blue Wall creature token with Defender, where X is the CMC of that spell.
{u}, {t}, Sacrifice a Wall: The next non-creature
spell you cast this turn has Flash.
{u}{u}{u}, {t}, Sacrifice five Walls: Take an extra
turn after this one.