Deal damage to Boros Reckoner
Fiendlash and Boros Reckoner trigger.
Resolve the Boros Reckoner trigger, dealing damage to any target
Resolve the Boros Reckoner trigger, dealing at least 1 damage to itself.
Repeat from step 2.

Infinite damage

Fiendlash

Boros Reckoner

Declare any creature as an attacker. Dawnsire triggers, dealing 100 damage to  the creature with Fiendlash attached
Fiendlash triggers, dealing 100 damage to target opponent.

Near-infinite damage to one opponent

Dawnsire, Sunstar Dreadnought

Deal damage to Barbed Servitor
Barbed Servitor and Fiendlash trigger.
Resolve the Barbed Servitor trigger, causing an opponent to lose life equal to the damage dealt to Barbed Servitor.
Resolve the Fiendlash trigger, targeting you, but dealing damage to Barbed Servitor instead due to Pariah's Shield.
Repeat from step 2.

Infinite damage to one opponent

Infinite lifeloss

Barbed Servitor

Pariah's Shield

Activate Pyrohemia by paying {R}, dealing 1 damage to each player and creature.
Fiendlash triggers, causing Blightsteel Colossus to give an opponent at least thirteen poison counters.
The opponent loses the game due to infect damage.
Repeat for each remaining opponent.

Combos with "Fiendlash"