Deal damage to Truefire Captain.
Truefire Captain and Blazing Sunsteel trigger.
Resolve the Truefire Captain trigger, dealing damage to any player.
Resolve the Blazing Sunsteel trigger, dealing damage to Truefire Captain.
Repeat from step 2.

Infinite damage

Blazing Sunsteel

Truefire Captain

Deal damage to Truefire Captain.
Truefire Captain triggers, dealing damage to an opponent.
Guilty Conscience triggers, dealing damage to Truefire Captain.
Repeat from step 2.

Infinite damage to target player

Guilty Conscience

Deal damage to Truefire Captain.
Truefire Captain and Fiendlash trigger.
Resolve the Truefire Captain trigger, dealing damage to an applicable target.
Resolve the Fiendlash trigger, targeting you, but dealing damage to Truefire Captain instead due to Pariah.
Repeat from step 2.

Fiendlash

Pariah

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

Repercussion

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

Near-infinite damage to target player

Dawnsire, Sunstar Dreadnought

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 Truefire Captain instead due to Pariah.
Truefire Captain triggers, dealing an arbitrarily large amount of damage to an applicable target.

Volcano Hellion

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

Near-infinite damage to target opponent

Wheel of Misfortune

Activate Tree of Perdition, exchanging target opponent's life total with Tree of Perdition's toughness.
Cast Blasphemous Act, dealing thirteen damage to all creatures.
Truefire Captain triggers, dealing thirteen damage to target opponent.

Target player takes a lethal amount of damage

Tree of Perdition

Blasphemous Act

Activate Tree of Perdition, exchanging target opponent's life total with Tree of Perdition's toughness.
Cast Star of Extinction for {5}{R}{R}, destroying target land and dealing 20 damage to all creatures and planeswalkers.
Truefire Captain triggers, dealing 20 damage to target opponent.

Star of Extinction

Give Truefire Captain indestructible, if needed.
Cast Chain of Plasma by paying {1}{R}, dealing 3 damage to Truefire Captain and causing you to discard a card and copy Chain of Plasma targeting Truefire Captain.
Taii Wakeen and Truefire Captain trigger.
Resolve the Taii Wakeen trigger, causing you to draw a card.
Resolve the Truefire Captain trigger, dealing 3 damage to any target.
Resolve the Chain of Plasma copy, dealing 3 damage to Truefire Captain and causing you to discard a card and copy Chain of Plasma targeting Truefire Captain.
Repeat from step 3.

Near-infinite damage

Infinite self-discard triggers

Infinite draw triggers

Near-infinite magecraft triggers

Infinite rummaging

Taii Wakeen, Perfect Shot

Chain of Plasma

Activate Tree of Perdition, exchanging target opponent's life total with Tree of Perdition's toughness.
Cast Shivan Meteor for {3}{R}{R}, dealing thirteen damage to Truefire Captain.
Truefire Captain triggers, dealing 20 damage to target opponent.

Shivan Meteor

Activate Tree of Perdition, exchanging target opponent's life total with Tree of Perdition's toughness.
Cast Into the Maw of Hell for {4}{R}{R}, destroying target land and dealing thirteen damage to Truefire Captain.
Truefire Captain triggers, dealing 20 damage to target opponent.

Into the Maw of Hell

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 Truefire Captain.
Truefire Captain triggers, dealing that much damage to an applicable target.
Resolve the Earthcraft trigger, untapping the earthbent land.
Repeat from step 3.

Combos with "Truefire Captain"