Greven does not have summoning sickness and cannot be blocked by an opponent.Your life total is at least 17.
({B} magic symbol) available.
Steps
Activate Blood Celebrant by paying ({B} magic symbol) and 1 life, causing you to add ({B} magic symbol) , and giving Greven +1/+1 until end of turn due to its first ability.
Repeat step 1 until Greven is a 21/21 creature.
Deal combat damage to an opponent using Greven.
The opponent loses the game due to being dealt 21 combat damage from your commander.