Sefris of the Hidden Ways, Radiant Solar, and Lagrella, the Magpie

Prerequisites

• Sefris of the Hidden Ways on the battlefield
• Radiant Solar on the battlefield
• Lagrella, the Magpie in hand
• available

Steps

1. Cast Lagrella by paying its mana cost
2. When Lagrella enters, it and Radiant Solar trigger
3. Resolve the Lagrella trigger, exiling Radiant Solar and two other nontoken creatures until Lagrella leaves the battlefield
4. Resolve the Radiant Solar trigger, venturing into the "Sandfall Cell" room, causing you to sacrifice Lagrella and each opponent to either sacrifice a creature, artifact or land or lose 2 life
5. Lagrella leaves the battlefield, returning Radiant Solar and the two other nontoken creatures from exile to the battlefield
6. Radiant Solar and the two creatures enter the battlefield, triggering Radiant Solar three times
7. Resolve the first Radiant Solar trigger, venturing into the "Cradle of the Death God" room, creating a 4/4 The Atropal legendary creature token, and completing Tomb of Annihilation
8. Sefris' second ability triggers, returning Lagrella from your graveyard to the battlefield
9. When Lagrella enters, it and Radiant Solar trigger
10. Resolve the Lagrella trigger, exiling Radiant Solar and two other nontoken creatures until Lagrella leaves the battlefield
11. Resolve the Radiant Solar trigger, venturing into the "Trapped Entry" room of the Tomb of Annihilation, causing each player to lose 1 life
12. Resolve the second Radiant Solar trigger from step 6, venturing into the "Veils of Fear" room, causing each player to lose 2 life unless they discard a card
13. Repeat from step 4

Results

• Near-infinite ETB
• Near-infinite LTB
• Near-infinite death triggers
• Near-infinite blinking
• Near-infinite lifeloss for all players