Card

Morophon, the Boundless

Mana cost {7}

Changeling (This card is every creature type.) As Morophon enters, choose a creature type. Spells of the chosen type you cast cost {W}{U}{B}{R}{G} less to cast. This effect reduces only the amount of colored mana you pay. Other creatures you control of the chosen type get +1/+1.

Part of 197 known combos, 4 of them two-card. 5 official rulings.

Commander: Legal
Printing: Mythic · CMM · #3
Cheapest printing: €4.23
EDHREC salt: 0.67
Color identity: Black, Green, Red, Blue, White
Keywords: Changeling

Anthem

Scryfall · EDHREC

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partners: Grinning Ignus, Shrieking Drake, Horned Kavu, Cavern Harpy
B2 Core — Not in the same deck as its two-card combo partners: Grinning Ignus, Shrieking Drake, Horned Kavu, Cavern Harpy
B3 Upgraded — allowed: Its two-card combos are not early, which Bracket 3 allows
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Why

Two-card combo with Grinning Ignus, Shrieking Drake, Horned Kavu, Cavern Harpy (none of them early)

How Lotus Vault estimates a bracket

Bracket: Lotus Vault analysis, with Commander Spellbook flags

Combos

197 known combos, 4 of them two-card.

Hibernation Sliver + Morophon, the Boundless + Syphon Sliver + Lavabelly Sliver
Produces: Infinite creature LTB, Infinite creature ETB, Infinite damage
View on Commander Spellbook
Morophon, the Boundless + Hibernation Sliver + Essence Sliver + Lavabelly Sliver
Produces: Infinite damage, Infinite creature ETB, Infinite lifegain triggers
View on Commander Spellbook
The Ur-Dragon + Haakon, Stromgald Scourge + Morophon, the Boundless + Nameless Inversion
Produces: Reduce the toughness of creatures opponents control to 0, Reduce the toughness of each creature opponents control to 0 upon entering
View on Commander Spellbook

Combos: Commander Spellbook

Rulings

2019-06-14 · Wizards of the Coast
If a spell has hybrid mana symbols in its mana cost, you choose which half you will be paying before determining the total cost. For example, if a spell of the chosen type costs {2}{W/U}{W/U}, you may choose for the cost to be {2}{W}{U} and then reduce it to {2}.
2019-06-14 · Wizards of the Coast
Morophon's effect reduces the total cost by up to one mana of each color. For example, if a spell of the chosen type costs {4}{R}{W}{W}, it will cost {4}{W} after applying Morophon's effect.
2019-06-14 · Wizards of the Coast
You must choose an existing creature type, such as Sliver or Warrior. Card types such as artifact, and supertypes such as legendary or snow, can't be chosen.
2019-06-14 · Wizards of the Coast
Cost reduction effects are applied after other cost modifiers, so Morophon can reduce additional costs or alternative costs of spells of the chosen type.
2019-06-14 · Wizards of the Coast
Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to other creatures you control of the chosen type may become lethal if Morophon leaves the battlefield during that turn.

Rulings: Scryfall · Wizards of the Coast