Hypnotic Siren is an interesting card. It’s not a lot different from Spirit Away when you cast it as an aura, except it only gives +1/+1 and it turns into a 1/1 Flyer if the stolen creature dies. Oh, and I suppose you could simply cast it on turn one as a 1/1 and start attacking (but we know that never really happens!).
What really makes this interesting is the existence of the new Constellation abilities. If you control a creature with a Constellation ability, it’s obvious what happens when you cast and resolve Hypnotic Siren… that ability triggers.
But what about when you bestow Hypnotic Siren on an opponent’s Eidolon of Blossoms, for example? Clearly an enchantment entered the battlefield under your control, but did you control Eidolon of Blossoms at the time? It gets a bit confusing, but the answer is that the Eidolon’s ability will trigger under your control, and you’ll draw a card.
Why? Well the nature of all triggered abilities is that immediately after a game event (like a card entering the battlefield), the game checks for any abilities that should trigger from that event. The important point here is that layers are checked continuously, so they don’t wait for priority or state-based actions. The Siren becomes attached, so you control the Eidolon, then the game checks for triggers. The game sees there’s an ability of a permanent you control that should trigger when the Siren enters under your control, so that ability triggers.
This isn’t an entirely new phenomenon. A similar situation could have occurred with Persuasion and Bramble Elemental. But Constellation being in the same set as Hypnotic Siren makes the interaction much more likely! There’s even a Gatherer ruling to address it:
If you Hypnotic Siren for its bestow cost, and you gain control of a creature with a constellation ability, that ability will trigger when Hypnotic Siren enters the battlefield. You’ll controll that triggered ability.
Today’s Rules Tip written by Josh Stansfield