Astarion's Thirst

Exile target creature. Put X +1/+1 counters on a commander creature you control, where X is the power of the creature exiled this way.

Bhaal, Lord of Murder

As long as your life total is less than or equal to half your starting life total, Bhaal, Lord of Murder has indestructible.
Whenever another nontoken creature you control dies, put a +1/+1 counter on target creature and goad it.

Bonecaller Cleric

{3}{B}, Sacrifice Bonecaller Cleric: Return target creature card from your graveyard to the battlefield. Activate only as a sorcery.

Karlach, Fury of Avernus

Whenever you attack, if it's the first combat phase of the turn, untap all attacking creatures. They gain first strike until end of turn. After this phase, there is an additional combat phase.
Choose a Background (You can have a Background as a second commander.)

Blood Money

Destroy all creatures. For each nontoken creature destroyed this way, you create a tapped Treasure token.

Bone Offering

Create a tapped 4/1 black Skeleton creature token with menace. (Then exile this card. You may cast the artifact later from exile.)

Altar of Bhaal

{2}{B}, {T}, Exile a creature you control: Return target creature card from your graveyard to the battlefield. Activate only as a sorcery.