First of Ancient Nightmares

When First of Ancient Nightmares enters the battlefield, each player sacrifices three
permanents. Put a number of +1/1 counters on
First of Ancient Nightmares equal to the number
of creature cards in all graveyards.
Whenever First of Ancient Nightmares dies, each player sacrifices three permanents. Then create 3
1/1 red Spawn of the First creature tokens. They
have, "Sacrifice this creature: Add {r} to your
mana pool."