Blood for the Blood God!

This spell costs {1} less to cast for each creature that died this turn.
Discard your hand, then draw eight cards. Blood for the Blood God! deals 8 damage to each opponent. Exile Blood for the Blood God!

Warhammer 40,000 Commander

This spell costs {1} less to cast for each creature that died this turn.
Discard your hand, then draw eight cards. Blood for the Blood God deals 8 damage to each opponent. Exile Blood for the Blood God

The ability that reduces the mana cost only reduces its generic mana cost. It will cost at least {B}{B}{R}, even if more than eight creatures have died this turn.