Siren of the Fanged Coast vs. Devotion

Question

Two of my friends and I were playing MTG and someone (we'll call her player 1) had played Siren of the Fanged Coast (SFC) and she was going to use its 'control creature' ability against one of player 2's god, which player 1 couldn't pay it's devotion cost to keep it as a creature.

My question is this: After player 1 ends her turn (while controlling the god) and it then became an enchantment would she still be in control of it? My reasoning is since SFC's ability isn't like Mind Control, where as that's a passive ability of the card, SFC's ability is a one-off thing. You officially have control of the god, it just so happens to become an enchantment shortly after you take it.

Answer 1

The fact that the ability says "gain control of target creature" only means that the chosen target must be a creature at the time it is chosen and at the time the ability resolves. This is just like any other targeted spell or ability: targets have to meet the specified conditions when you choose them, and also when the spell or ability resolves.

After the ability resolves, it doesn't matter what happens to the targeted god's characteristics - whether it stops being a creature, changes color, starts being a creature again, changes power and toughness, gains or loses abilities, or so on. Nothing causes the control-changing effect to end, unless it has its own built-in time limit (for things like Act of Treason). Of course it can be overridden by another control-changing effect, i.e. player 2 could steal the god back. Here's the relevant rule:

611.2c. If a continuous effect generated by the resolution of a spell or ability modifies the characteristics or changes the controller of any objects, the set of objects it affects is determined when that continuous effect begins. After that point, the set won't change. (Note that this works differently than a continuous effect from a static ability.) A continuous effect generated by the resolution of a spell or ability that doesn't modify the characteristics or change the controller of any objects modifies the rules of the game, so it can affect objects that weren't affected when that continuous effect began. If a single continuous effect has parts that modify the characteristics or changes the controller of any objects and other parts that don't, the set of objects each part applies to is determined independently.

You're basically right about the difference between this and Mind Control, by the way. With Mind Control, the control-changing effect comes from a static ability of a permanent, as opposed to the Siren's effect which comes from the resolution of an ability. The relevant rule for Mind Control is this one:

611.3b. The effect applies at all times that the permanent generating it is on the battlefield or the object generating it is in the appropriate zone.

So in that case, the control-changing effect only lasts as long as the Mind Control is actually enchanting the thing.

Answer 2

Since the tribute wasn't paid for SFC, the if statement of the Come into Play ability is satisfied. Player 1 gains control of the God, but only if it is a legal target, i.e. Player 2 has devotion.

When Player 1 gains control of the god, since she doesn't have devotion it stops being a creature. However, she retains control of the God since the ability doesn't say "until end of turn" Act of Treason.

In the case of Act of Treason, or similar, against a Theroes God, the duration still applies even if it is no longer a creature. At end of turn, it will return to its previous controller.