My first instinct was that this seems really annoying, but I guess it isn't that powerful when compared to Awakening Zone or such.
EDIT:
After some though, this is can easily give you 10+ tokens at a multiplayer game of 3+ players. That's gotta be busted.
Some suggestions:
mana cost : ~ costs more to cast for each opponent.
mana cost : As ~ ETBs, choose an opponent. Whenever the chosen opponent, ...
mana cost or mayyybe: No changes.
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My first instinct was that this seems really annoying, but I guess it isn't that powerful when compared to Awakening Zone or such.
EDIT:
After some though, this is can easily give you 10+ tokens at a multiplayer game of 3+ players. That's gotta be busted.
Some suggestions:
mana cost
: ~ costs 
more to cast for each opponent.
: As ~ ETBs, choose an opponent. Whenever the chosen opponent, ...
or 
mayyybe: No changes.
mana cost
mana cost