A Monument to All Your Sins

Where x is fifteen minus the total number of creatures on the battlefield and in all graveyards
As an additional cost to summon, you MUST sacrifice Gravemind
Hexproof, Indestructible
Your creatures gain +2/+2, Lifelink, and Double-Strike when attacking sentient creatures
All creatures sent to the graveyard are
immediately returned to the battlefield under
your control