Say hello to the newset three companions in the discussion.
Jegantha can be run in 5-color decks that accommodate it. Just not ones that run something like Worldpurge or Kaya's Wrath. Probably great for sliver tribal decks like Morophon, the Boundless which rarely if never has more than one of the same mana symbol in their casting cost
Zirda only cares if your starting deck has permanents with activated abilities. Gives you Training Grounds for activated abilities, and even applies to ones that are not on the battlefield. Eternal Dragon for example has an activated ability that would be affected by this ability that can only be activated during the upkeep, but it also has a cycling ability which is an activated ability. Making the overall cost to recur this dragon and cycle it again for another plains card just 2WW instead of 5WW, and if you were willing to pay an extra 1 on top of the effective original cost, you can get two plains cards such as a Savannah and a Hallowed Fountain.
Kaheera is obviously more about tribal and is less explosive, but it already has a home in decks such as Arahbo, Roar of the World.
11. Abilities which bring card(s) you own from outside the game into the game (such as Living Wish; Spawnsire of Ulamog; Karn, the Great Creator) do not function in Commander.
Quote from Wizards Website »
There are ten companions in Ikoria, each one a legendary creature whose companion ability lists a deck-building rule. That chosen companion doesn't start in your main deck. Rather, it's a card in your sideboard. (If you're playing casually without sideboards, it's just in your collectionoutside the game. All the same rules apply to it.)
So it makes you wonder what they have in store if they allow companions when it would otherwise just get blanked by rule 11.
Jegantha can be run in 5-color decks that accommodate it. Just not ones that run something like Worldpurge or Kaya's Wrath. Probably great for sliver tribal decks like Morophon, the Boundless which rarely if never has more than one of the same mana symbol in their casting cost
Zirda only cares if your starting deck has permanents with activated abilities. Gives you Training Grounds for activated abilities, and even applies to ones that are not on the battlefield. Eternal Dragon for example has an activated ability that would be affected by this ability that can only be activated during the upkeep, but it also has a cycling ability which is an activated ability. Making the overall cost to recur this dragon and cycle it again for another plains card just 2WW instead of 5WW, and if you were willing to pay an extra 1 on top of the effective original cost, you can get two plains cards such as a Savannah and a Hallowed Fountain.
Kaheera is obviously more about tribal and is less explosive, but it already has a home in decks such as Arahbo, Roar of the World.