Card

Archon of Emeria

Mana cost {2}{W}

Flying Each player can't cast more than one spell each turn. Nonbasic lands your opponents control enter tapped.

Part of 4 known combos, 4 of them two-card. 3 official rulings.

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partners: Knowledge Pool, Possibility Storm, Eye of the Storm, Erayo, Soratami Ascendant // Erayo's Essence
B2 Core — Not in the same deck as its two-card combo partners: Knowledge Pool, Possibility Storm, Eye of the Storm, Erayo, Soratami Ascendant // Erayo's Essence
B3 Upgraded — Not in the same deck as its early two-card combo partners: Knowledge Pool, Possibility Storm, Eye of the Storm
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Two-card combo with Knowledge Pool, Possibility Storm, Eye of the Storm, Erayo, Soratami Ascendant // Erayo's Essence (early with Knowledge Pool, Possibility Storm, Eye of the Storm)

Bracket: Lotus Vault analysis, with Commander Spellbook flags

4 known combos, 4 of them two-card.

Archon of Emeria + Knowledge Pool
Produces: Exile all spells players cast from their hand, Players can only cast one spell per turn, Lock
View on Commander Spellbook
Possibility Storm + Archon of Emeria
Produces: Players can't cast spells from their hand, Lock
View on Commander Spellbook
Archon of Emeria + Eye of the Storm
Produces: Exile all instants and sorceries any player casts, Players can only cast one spell per turn, Lock
View on Commander Spellbook

2020-09-25 · Wizards of the Coast
If an effect says that a land enters tapped unless a condition is met or a cost is paid, that land enters tapped even if that condition is met or that cost is paid if it's affected by Archon of Emeria's second ability.
2020-09-25 · Wizards of the Coast
If you cast a spell that was countered, you can't cast another spell during the same turn.
2020-09-25 · Wizards of the Coast
Archon of Emeria looks at the entire turn to see if a player has cast a spell, even if Archon of Emeria wasn't on the battlefield when that spell was cast. Notably, you can't cast Archon of Emeria and then cast another spell during the same turn.

Rulings: Scryfall · Wizards of the Coast