Activate Fanatical Firebrand by tapping and sacrificing it.
When Fanatical Firebrand dies, Mortuary and Vindictive Vampire trigger.
Resolve the Mortuary trigger, putting Fanatical Firebrand from your graveyard onto the top of your library.
Resolve the Vindictive Vampire trigger, dealing 1 damage to each opponent and causing you to gain 1 life.
Resolve the Fanatical Firebrand ability.
Cast Fanatical Firebrand from the top of your library by paying 1 life.
Repeat.
This clever loop turns a humble Fanatical Firebrand into an endless barrage of sacrifice and damage. While Vindictive Vampire passively watches every creature die to drain your opponents' life totals, Mortuary ensures your sacrificed goblin never truly stays gone, immediately recycling it to the top of your library. From there, Gwenom, Remorseless allows you to bypass mana costs entirely by casting the Firebrand straight from the top of your deck, using its lifelink to comfortably pay the life needed to fuel the cycle.