Zero Sum

[Daenyth]

Zero Sum   
Legendary Enchantment
Whenever a player would gain life, that player chooses another target player to lose an equal amount of life.
Whenever a player would lose life, if the source of the life loss is controlled by another player, that source's controller gains an equal amount of life.

[Delha]

Neat concept, but isn't this strictly inferior to Sanguine Bond?

[Daenyth]

Huh, I didn't know about that card. Nifty. The difference here is that at the same time, damaging your opponent will also gain you life at the same time.

[Delha]

Sorry, I misread the second line. I honestly don't remember what I thought it said.

This card is a self contained infinite loop. Triggering the first ability will set off the second, which will in turn trigger the first again. The "if controlled by" clause (which I don't understand the point of) stops it from going infinite on you, coincidentally.

Were you trying to make these replacement effects so that they wouldn't chain like that? The syntax leads me to believe you were.

[Daenyth]

They're not intended to loop, but I couldn't figure out the templating. I think it would have to be "Whenever a player gains or loses...", but that's incredibly clunky.

[Delha]

How about the below?

Whenever a source other than Zero Sum causes a player to gain life, that player chooses a target opponent. That opponent loses an equal amount of life.

Whenever a source other than Zero Sum causes a player to lose life, that player chooses a target opponent. That opponent gains an equal amount of life.

Note that two copies will still generate a loop, repeating whatever action initiates it. Things get potentially get complicated if an effect makes multiple people gain/lose at once, and different people control the copies. Things can also get weird if there are more than two players.

[Daenyth]

Duh, why didn't I think of that. Sounds good