Grok, Lord of Pebbles

Whenever you play a creature with defender, if
Grok, Lord of Pebbles is in your command zone,
you may put him onto the battlefield.
Defender, Grok, Lord of Pebbles enters the battlefield with a +1/+1 counter for each mana
spent to cast him.
A rolling stone gathers no moss, it gathers
more rocks... I think.