Card

Teferi, Mage of Zhalfir

Mana cost {2}{U}{U}{U}

Flash Creature cards you own that aren't on the battlefield have flash. Each opponent can cast spells only any time they could cast a sorcery.

Part of 52 known combos, 3 of them two-card. 5 official rulings.

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partners: Knowledge Pool, Omen Machine, Possibility Storm
B2 Core — Not in the same deck as its two-card combo partners: Knowledge Pool, Omen Machine, Possibility Storm
B3 Upgraded — Not in the same deck as its early two-card combo partners: Knowledge Pool, Omen Machine, Possibility 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, Omen Machine, Possibility Storm (early with Knowledge Pool, Omen Machine, Possibility Storm)

Bracket: Lotus Vault analysis, with Commander Spellbook flags

52 known combos, 3 of them two-card.

Teferi, Mage of Zhalfir + Knowledge Pool
Produces: Exile all spells opponents cast from their hands, Lock
View on Commander Spellbook
Teferi, Mage of Zhalfir + Omen Machine
Produces: Opponents can only play lands exiled by Omen Machine, Players can't draw cards, Lock
View on Commander Spellbook
Teferi, Mage of Zhalfir + Possibility Storm
Produces: Exile all spells opponents cast from their hands, Opponents can't play cards exiled by Possibility Storm, Lock
View on Commander Spellbook

2021-03-19 · Wizards of the Coast
Teferi's last ability means that in order for an opponent to cast a spell, it must be that opponent's turn, during a main phase, and the stack must be empty. This is true even if a rule or effect allows a sorcery to be cast at another time.
2021-03-19 · Wizards of the Coast
If an effect allows an opponent to cast spells any time they could cast an instant (for example, if your opponent also controls Teferi), the restriction of Teferi's last ability takes precedence over that permission.
2021-03-19 · Wizards of the Coast
If a spell on the stack has split second while you control Teferi, you can't suspend creature cards since split second stops you from casting them, even though suspending them is a special action.
2021-03-19 · Wizards of the Coast
While you control Teferi, you may exile creature cards in your hand with suspend any time you could cast an instant.
2021-03-19 · Wizards of the Coast
If a spell or ability lets an opponent cast a spell as part of its effect (such as suspend and rebound do), that opponent can't cast that spell since the resolving ability is still on the stack. This is true even if the spell is an instant.

Rulings: Scryfall · Wizards of the Coast