Does Selvala Heart of the Wilds trigger for each opponent's creature that enters or only your own?

Selvala, Heart of the Wilds has a triggered ability that reads: 'Whenever a creature enters the battlefield, its controller may have Selvala add mana equal to that creature's power.' There is no restriction limiting this to creatures you control — the trigger watches for any creature entering the battlefield, regardless of who controls it.

Under CR 603.1 and 603.4, triggered abilities use the word 'whenever' and trigger based on the specified event occurring anywhere the ability can watch. Since Selvala's ability doesn't include the phrase 'you control' in its trigger condition, it fires for every creature that enters, including opponents' creatures.

The key nuance is that the controller of the entering creature decides whether Selvala adds mana — not Selvala's controller. So if an opponent's 5/5 enters, that opponent chooses whether to tap Selvala for 5 mana. Selvala's controller does not get the mana in that case.

Concrete example: It's your opponent's turn and they play Craterhoof Behemoth (8 power). Selvala triggers. Your opponent, as the Craterhoof's controller, may choose to have Selvala add 8 green mana — but that mana goes to them, not you. You simply provided the mana source via Selvala.

This makes Selvala a politically powerful card in Commander (CR 800.4 covers multiplayer), as opponents may want to trigger her and can benefit from doing so, sometimes leading to cooperative or tense group dynamics around her ability.