Gaining control of a creature while an Aura targeting it is on the stack

Question

It's my opponent turn. I have Prophet of Kruphix on the battlefield, which means I can play creatures as though they had flash.

My opponent plays Boonweaver Giant and chooses Spectra Ward, which goes on the stack as triggered ability (?)

I play Hypnotic Siren with bestow to gain control of Boonweaver Giant in response to the triggered ability. (I can do this during my opponent's turn because I control Prophet of Kruphix.)

Does that mean that the stack looks like this?

        TOP
    Hypnotic Siren   \
                     |
  / Spectra Ward     | 1
2 |                  |
  \ Boonweaver Giant /

        BOTTOM

Will I gain control of Boonweaver Giant, and then it gets the Spectra Ward Aura under my control?

Can someone tell me whether there is a mistake in this play?

The rulings on Hypnotic Siren say:

Gaining control of a creature doesn’t cause you to gain control of any Auras or Equipment attached to it.

which does not apply to this situation because Boonweaver Giant does not have any Auras attached to it when I gain control of it with Hypnotic Siren.

Answer 1

First, if Boonweaver Giant's triggered ability is on the stack, then Boonweaver himself is not on the stack. He has already entered the battlefield. Second, Spectra Ward should never have been placed on the stack. Boonweaver Giant's triggered ability is what actually gets placed on the stack. Your opponent doesn't have to make a decision until it resolves.

With these corrections in mind, here is what your stack actually looks like:

• Hypnotic Siren (TOP)
• Boonweaver Giant's triggered ability (BOTTOM)

Hypnotic Siren resolves. You gain control of Boonweaver Giant. Next, Boonweaver Giant's ability resolves. Your opponent may search for an aura and attach it to Boonweaver Giant. He may choose not to. If he does, he will control the aura attached to Boonweaver, and you will control Boonweaver.

Answer 2

Casting Hypnotic Siren on Boonweaver Giant will not give you control of Spectra Ward. However, this doesn't matter, as Spectra Ward will continue to give its bonus to Boonweaver Giant anyway, and (unlike, say, Curiosity) this bonus does not depend in any way on who controls Spectra Ward.

Also, your understanding of how the stack works here is flawed. If you intend to cast Hypnotic Siren after the Spectra Ward enters the battlefield, then the correct order of events is:

1. Your opponent casts Boonweaver Giant.

2. Boonweaver Giant resolves and enters the battlefield (and thus is no longer on the stack).

3. Boonweaver Giant's "enters the battlefield" ability triggers and goes on the stack. Your opponent does not search for an Aura at this time.

4. Boonweaver Giant's ETB trigger resolves. At this point, your opponent searches his or her library for an Aura and puts it onto the battlefield attached to Boonweaver Giant. This happens all at once. The Aura is never on the stack.

5. Due to the Spectra Ward, Boonweaver Giant now has protection from blue, and so you cannot cast Hypnotic Siren on it.

If you instead cast Hypnotic Siren before Spectra Ward enters the battlefield, that will have to take place immediately after step 3. At that point, you will have no knowledge of what Aura your opponent will search for, and by the time your opponent does search, you will control the Boonweaver Giant and (because its ability says "may") your opponent (who still controls the ETB ability) has the option to not search for an Aura to place on the Giant — or to search for an Aura that negatively affects you (like a Control Magic to steal the Giant back!).

Answer 3

You cannot bestow enchantments during your opponents turn. When you bestow an enchantment creature it is considered an aura. Prophet only allows you to cast creature spells.

Card Text: Bestow 5BlueBlue (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.) Flying You control enchanted creature. Enchanted creature gets +1/+1 and has flying.