Void Rift

Whenever enchanted land is tapped for mana, put a
void counter on that land.
Enchanted land has "{t}, Remove X void counters
from this land: Add an amount of mana of any color
equal to X to your mana pool. This mana can only be
used to cast Void creatures. If that creature is
legendary, sacrifice Void Rift."
"For the Void." —Malzahar, the Prophet of the Void