Endrek Sahr, Master Breeder

Whenever you cast a creature spell, create X 1/1 black Thrull creature tokens, where X is that spell's mana value.
When you control seven or more Thrulls, sacrifice Endrek Sahr, Master Breeder.

Commander Masters

The List

Modern Masters 2015

Whenever you cast a creature spell, put X 1/1 black Thrull creature tokens onto the battlefield, where X is that spell's converted mana cost.
When you control seven or more Thrulls, sacrifice Endrek Sahr, Master Breeder.

Commander 2013

Face-down spells are creature spells, but they have mana value 0.

The second ability is a state trigger. Once it triggers, it won’t trigger again as long as the ability is on the stack. If the ability is countered and the trigger condition is still true, it will immediately trigger again.

When the second ability resolves, Endrek Sahr will be sacrificed regardless of how many Thrulls you control at that time, even if it’s less than seven.