Some have speculated that you can't play this in Legacy, but I think that you can firstly calculate the number (zero) and then pay that much life, so it should be fine

Except your Commander's color identity isn't zero, it's non-existent. Those terms are already differentiated in the rules when it comes to Mana cost (see memnite vs. evermind), so I think we'll get a similar ruling here.

The CMC of Evermind is 0, though. If you have Maelstrom Nexus and play Llanowar Elves first you can cascade equally to Memnite or Evermind.
The number of colors in {NULL SET} is also 0.

Wish this had a Maze of Ith effect just for the Dr. Strangelove joke.

I feel like it would be an undefined set, not a null set. You can't actually determine the number of colors of something which doesn't exist.

I wouldn't rule out that being a ruling, but I think it would need a new ruling. Possibly a sweeping one locking out the commander and other format-locked cards. Right now though we can look at Evermind or the costless suspends not having a mana cost and say "The amount of mana in a mana cost that doesn't exist is 0", so I don't see how we wouldn't say "The amount of colors in a set of colors that doesn't exist is 0"

I don't know, so I'm mostly doing this as a thought experiment, but I could argue that converted Mana cost creates an exception which does not exist for color identities. Yes, evermind has cmc 0 and can be affected but things that care about cmc, but Embodiment of Agonies can't count it, because it has no Mana cost. War Room doesn't ask for a "converted color identity", it asks for a color identity.

I wonder if we can argue from the point of view of "why did they make this card". Could have just been a fixed amount of life. Or, if they wanted it to be no life for legacy, they could've made it no life for commander too. So is this wording to keep it out of legacy? Or just to benefit commander decks with less colors?

It is definitely to benefit commanders work fewer colors. This is going to be one of the best card draw engines in mono white, but not nearly as good in most other decks. It's a really clever way to balance the format a bit.

War Room can only be used for the T: Add C effect in formats without a commander, as you can't pay nonexistent costs and the life it would take isn't 0 it's nonexistent.

https://twitter.com/gavinverhey/status/ ... 2053915649