Card

Mesmeric Orb

Mana cost {2}

Whenever a permanent becomes untapped, that permanent's controller mills a card.

Part of 34 known combos, 5 of them two-card. 1 official ruling.

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partners: Basalt Monolith, Aphetto Alchemist, Seeker of Skybreak, Tidewater Minion, Ith, High Arcanist
B2 Core — Not in the same deck as its two-card combo partners: Basalt Monolith, Aphetto Alchemist, Seeker of Skybreak, Tidewater Minion, Ith, High Arcanist
B3 Upgraded — Not in the same deck as its early two-card combo partners: Basalt Monolith, Aphetto Alchemist, Seeker of Skybreak, Tidewater Minion, Ith, High Arcanist
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Two-card combo with Basalt Monolith, Aphetto Alchemist, Seeker of Skybreak, Tidewater Minion, Ith, High Arcanist (early with Basalt Monolith, Aphetto Alchemist, Seeker of Skybreak, Tidewater Minion, Ith, High Arcanist)

Bracket: Lotus Vault analysis, with Commander Spellbook flags

34 known combos, 5 of them two-card.

2020-08-07 · Wizards of the Coast
If permanents become untapped during the untap step, Mesmeric Orb's ability will trigger that many times. However, since no player gets priority during the untap step, those abilities wait to be put on the stack until the upkeep starts. At that time, any "beginning of upkeep" triggers will also trigger. Those abilities and Mesmeric Orb's triggers are put onto the stack at the same time.

Rulings: Scryfall · Wizards of the Coast