Cast Star of Extinction by paying {5}{R}{R}, dealing 20 damage to each creature and planeswalker and destroying a land.
Each Sliver you control triggers, dealing 20 damage to any target for each trigger.

Near-infinite damage

Spiteful Sliver

Star of Extinction

Cause Spiteful Sliver to take damage by any method.
Spiteful Sliver triggers, dealing damage to an opponent.
Guilty Conscience triggers, dealing damage to Spiteful Sliver.
Repeat from step 2.

Infinite damage to one opponent

Card that gives indestructible

Guilty Conscience

Declare any creature as an attacker. Dawnsire triggers, dealing 100 damage to  Spiteful Sliver.
Spiteful Sliver triggers, dealing 100 damage to target player.

Near-infinite damage to target player

Dawnsire, Sunstar Dreadnought

Deal damage to Spiteful Sliver.
Spiteful Sliver and Repercussion trigger.
Resolve the Spiteful Sliver trigger, dealing damage to an applicable target.
Resolve the Repercussion trigger, dealing damage to Spiteful Sliver instead of you due to Pariah.
Repeat from step 2.

Repercussion

Pariah

Cast Volcano Hellion by paying {2}{R}{R}.
When Volcano Hellion enters, it triggers, dealing an arbitarily large amount of damage to you and a creature. The damage that would be dealt to you is dealt to Spiteful Sliver instead due to Pariah.
Spiteful Sliver triggers, dealing an arbitrarily large amount of damage to an applicable target.

Volcano Hellion

Cast Fire Covenant by {1}{B}{R} and paying an amount of life equal to an opponent's life total, dealing that much damage to Spiteful Sliver.
Spiteful Sliver triggers, dealing that much damage to the opponent.

Fire Covenant

Cast Wheel of Misfortune by paying {2}{R}, choosing an arbitrarily large number and having Wheel of Misfortune deal that much damage to Spiteful Sliver instead of you due to Pariah if you chose the largest number.
Spiteful Sliver triggers, dealing that much damage to an applicable target.

Near-infinite damage to one opponent

Wheel of Misfortune

Earthbend a basic land you control.
Cast Legolas's Quick Reflexes by paying {G}, targeting the earthbent land.
Activate Earthcraft by tapping the earthbent land, targeting the earthbent land.
The land triggers, causing it to deal damage equal to its power to Spiteful Sliver.
Spiteful Sliver triggers, dealing that much damage to an applicable target.
Resolve the Earthcraft trigger, untapping the earthbent land.
Repeat from step 3.

Combos with "Spiteful Sliver"