Card

Ob Nixilis, the Hate-Twisted

Mana cost {3}{B}{B}

Whenever an opponent draws a card, Ob Nixilis deals 1 damage to that player. −2: Destroy target creature. Its controller draws two cards.

Part of 10 known combos, one of them two-card. 2 official rulings.

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partner: Peer into the Abyss
B2 Core — Not in the same deck as its two-card combo partner: Peer into the Abyss
B3 Upgraded — allowed: Its two-card combos are not early, which Bracket 3 allows
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Bracket: Lotus Vault analysis, with Commander Spellbook flags

10 known combos, one of them two-card.

Ob Nixilis, the Hate-Twisted + Peer into the Abyss
Produces: Target player loses the game
View on Commander Spellbook
Vilis, Broker of Blood + Ob Nixilis, the Hate-Twisted + Harmless Offering
Produces: Infinite card draw for target opponent, Infinite draw triggers for target opponent, Near-infinite damage to one opponent
View on Commander Spellbook
Vilis, Broker of Blood + Assault Suit + Ob Nixilis, the Hate-Twisted
Produces: Each opponent loses the game, Infinite card draw for each opponent, Infinite draw triggers for each opponent
View on Commander Spellbook

2019-05-03 · Wizards of the Coast
If the target creature is an illegal target by the time Ob Nixilis's last ability tries to resolve, the ability doesn't resolve. No player draws two cards. If the target is legal but not destroyed (most likely because it has indestructible), its controller does draw two cards.
2019-05-03 · Wizards of the Coast
If Ob Nixilis leaves the battlefield before his last ability resolves, most likely because he only had 2 loyalty when you activated the ability, his first ability won't exist to trigger when the creature's controller draws two cards.

Rulings: Scryfall · Wizards of the Coast