Card

Spiteful Sliver

Mana cost {2}{R}

Sliver creatures you control have "Whenever this creature is dealt damage, it deals that much damage to target player or planeswalker."

Part of 14 known combos, 3 of them two-card. 3 official rulings.

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partners: Star of Extinction, Dawnsire, Sunstar Dreadnought, Fire Covenant
B2 Core — Not in the same deck as its two-card combo partners: Star of Extinction, Dawnsire, Sunstar Dreadnought, Fire Covenant
B3 Upgraded — Not in the same deck as its early two-card combo partners: Dawnsire, Sunstar Dreadnought, Fire Covenant
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Two-card combo with Star of Extinction, Dawnsire, Sunstar Dreadnought, Fire Covenant (early with Dawnsire, Sunstar Dreadnought, Fire Covenant)

Bracket: Lotus Vault analysis, with Commander Spellbook flags

14 known combos, 3 of them two-card.

2019-06-14 · Wizards of the Coast
Damage dealt by the Sliver due to its triggered ability isn’t combat damage, even if it was combat damage that caused that ability to trigger.
2019-06-14 · Wizards of the Coast
If a Sliver is dealt damage by multiple sources at once (such as by two creatures blocking it), its ability triggers once and one target is dealt that much damage.
2019-06-14 · Wizards of the Coast
The ability granted by Spiteful Sliver will trigger even if the Sliver is dealt lethal damage. For example, if a 2/2 Sliver blocks a 7/7 creature, the Sliver’s ability will trigger and it will deal 7 damage.

Rulings: Scryfall · Wizards of the Coast