Activate Altar of Dementia by sacrificing the Goblin, Orc or Army
When the Goblin, Orc or Army dies, The Great Goblin's second ability and Mortuary trigger.
Resolve the Mortuary trigger, putting the Goblin, Orc or Army onto the top of your library from your graveyard.
Resolve the Great Goblin trigger, exiling the Goblin, Orc or Army from the top of your library with the ability to play it until the end of your next turn.
Resolve the Altar of Dementia ability
Cast the Goblin, Orc or Army from exile by paying its mana cost.
Birgi triggers, adding
Repeat
This clever loop locks your opponents in a doom of endless cycles by turning a single creature's death into an eternal loop of return and sacrifice. When you feed your chosen creature to Altar of Dementia, Mortuary steps in to instantly rescue it from the graveyard by placing it right back on top of your library. That exact journey is seized by The Great Goblin, which both exiles the creature to your hand-equivalent zone for casting and rewards your graveyard triggers. Every time you recast that creature, Birgi, God of Storytelling // Harnfel, Horn of Bounty floods your mana pool with red energy to sustain the cost, letting you mill out the table with Altar of Dementia while fueling a boundless storm count.