Activate Wall of Blood by paying 1 life.
Repeat step 1 until Greven has power 21 or greater due to its first ability.
Deal combat damage to an opponent using Greven. The opponent loses the game due to being dealt 21 or more combat damage from your commander.

Target opponent loses the game

Greven, Predator Captain

Wall of Blood

Cast Fire Covenant by paying {1}{B}{R} and 16 life, dealing 16 damage divided among any number of creatures and causing Greven to become a 21/21 until end of turn due to its first ability.
Deal combat damage to an opponent using Greven.
The opponent loses the game due to being dealt 21 combat damage from your commander.

Combos with "Greven, Predator Captain"