Resolve Phantom Steed's trigger, exiling Karmic Guide until Phantom Steed leaves the battlefield.
Resolve Radiant Solar's trigger, venturing into the Sandfall Cell room of Tomb of Annihilation.
Sandfall Cell triggers twice.
Resolve one Sandfall Cell trigger, sacrificing Phantom Steed and causing each other player to lose 2 life unless they sacrifice an artifact, creature, or land.
Karmic Guide enters the battlefield as Phantom Steed's effect ends.
Radiant Solar and Karmic Guide trigger.
Resolve Karmic Guide's trigger, returning Phantom Steed from your graveyard to the battlefield.
Radiant Solar and Phantom Steed trigger.
Resolve Phantom Steed's trigger, exiling Karmic Guide until Phantom Steed leaves the battlefield.
Resolve the Radiant Solar trigger from step 10, venturing into the Cradle of the Death God room.
Cradle of the Death God triggers twice, creating two legendary tokens of The Atropal.
Put one of The Atropal tokens into the graveyard due to the legend rule.
Resolve the Radiant Solar trigger from step 8, venturing into the Trapped Entry room of Tomb of Annihilation.
Trapped Entry triggers twice, causing each player to lose a total of 2 life.
Resolve the remaining Sandfall Cell trigger from step 5, sacrificing Phantom Steed and causing each other player to lose 2 life unless they sacrifice an artifact, creature, or land.
Repeat steps 7 through 11.
Resolve one of Radiant Solar's triggers, venturing into the Veils of Fear room.
Veils of Fear triggers twice, causing each player to lose 2 life unless they discard a card, twice.
Repeat from step 4 as many times as the game state allows, choosing a different target in step 3 or 9 to end the loop if necessary.