The target remained legal, hence why the spell resolved as a mutate. They can take control of any non-human creature you control that way, yes. Taking control of the spell did not change how the spell works, it mutated onto the targeted creature just like it was supposed to. They could only target your creatures, because they did not own the spell they took control over, you did. Nor did they control a creature you owned.

Control over the permanent changed at the moment the mutate spell resolved and merged with the permanent. The mutate triggers were checked after that, at which point they controlled the source and thus the triggers. They are also not obliged to change the target of the spell, they can keep the original target.

Okay so why did i get two for oned with their control spell? And if they chose a different target with my mutate spell is that a quasi-gilded drake effect? Can they chose a different creature i control and take control of that? It seems really inconsistent to me. Why does taking control or the spell change how the spell works. I can mutate onto an opponents creature if i own it. so they should be able to do the same even if they gained control of it.

What i meant is its not tied to the permanent in question until it has actually mutated onto the creature. And there were legal targets, that interaction is such a blowout to someone playing mutate its not even funny.

I don't quite understand what you are saying there. The mutate spell resolving as a creature spell and entering as a creature when the target becomes illegal, was made that way to negate the inherrent 2-for-1 it otherwise would create in the set centered in large part around mutate. You'd still get something when the target is removed. Note, that the change to a mere creature spell happens as the spell tries resolves, not beforehand.

But its not on the permanent yet its still on the stack. Why make the rule if you kill whatever creature you try to mutate onto it becomes a creature, if you are not showing that the creature you are mutating isnt on the creature you are targeting until the spell resolves.

I stated my take on that below. The reason is the control change effect carrying over to the permanent, because the mutate spell IS a permanent spell. When it merges with the permanent, the control change effect affects the entire permanent. If they changed the target, then they would gain control of the new target. When casting a spell an opponent owns yourself, there is no control change effect. They are the default controller, because they put the spell on the stack. With no control change effect, the merged permanent only has its default controller, which does not change when the mutate spell resolves.

Okay so why does controling the mutate spell take the permanent. They took it on the stack before the creature got mutated onto. If they were to change targets to another one of my creatures would they also get that creature. Why does gaining control of the spell work differently than if that person were to cast it themselves

Storm is a trigger, that goes off as soon as the spell is cast. There is no window to take control of the spell in time to also get control of the trigger. The trigger is controlled by the player who controlled the spell at the time storm triggered. Which is the original caster, since the trigger happens DURING the casting process.

A bestowed enchantment becomes attached to the permanent, it does not become part of it, unlike mutate. Controlling an aura does not give control of the permanent it is attached to (unless that is the effect of the aura).

Okay then why when you take control of a storm spell you only get the original spell and not all of the storm triggers. It seems like there is a special mutate rule going on or a bug happened. It would also mean that if my opponent took control of a bestow spell then they would get whatever creature they put the bestow spell onto.

Taking control of a spell creates a change of control effect, which carries over to the permanent the spell becomes (if it is a permanent spell). Casting a spell that you do not own just makes the player the default controller, no control change effect is created, and thus no control change effect can carry over to the permanent.

Still waiting on any responses in the rules forum.

Yeah but if they were to use heist they would still have a legal target. Why when they cast a spell that takes control of my spell the rules work differently than if they themselves were to cast the spell. Isnt taking control of a spell the same as if they were to cast the spell? That seems like such a different outcome, that there should be something in the rules that states that.

With heist, there is no control change effect, so everything stays with the default controller. Since the mutate spell doesn't enter the battlefield but merges with the target, the default controller is still the player who controls the creature that's mutated onto.

With a control change effect, though, that carries over to the permanent.

Still waiting on answers in the rules forum, though. The above is my take on the situation.

Yeah and i put the mutate creature on the stack so shouldnt they have only been able to choose my creature, and they get to choose whats on top. Because that would mean if someone steals one of my cards (with heist or something) then mutate onto my creature they would gain control of my creature. But thats not what happens. Same with if they use mass manipulation to steal my creature I wouldnt get it back if i mutate onto it i just give them a bunch of triggers and chose whats on top.

My bad, the rule (702.140a) says OWNER, not CONTROLLER. And ownership doesn't change. Thus the targeted creature is still a legal target, so when the mutate spell resolves, the CONTROLLER of the spell chooses where in the mutate stack it goes (top or bottom), they become the CONTROLLER of the entire mutate stack. With that, they control all the triggers as well. I think, the below rule is the relevant one, becasue a mutate spell is still a permanent spell.

110.2b If an effect causes a player to gain control of another player’s permanent spell, the first
player controls the permanent that spell becomes, but the permanent’s controller by default is
the player who put that spell onto the stack. (This distinction is relevant in multiplayer games;
see rule 800.4c.)

I made a thread in a rule forum I frequent, to gain more insight and possible correction. Will post any results, if the people answering don't do so themselves here.

They took the entire stack of creatures and got the all the mutate triggers as well. I had a cavern whisperer and when they gained control of my snapdax I ended up losing cavern whisperer(was mutated onto the snapdax) and i had to discard a card. Also then, why when my opponent steals my card (with heist for example) and casts it they can only target a creature I control. And thats still a legal target, my own creature. If an opponent uses act of treason on my creature I can mutate onto the creature on their side. And if i steal a mutate card im only able to target my opponents creatures or cast it as a creature. And they also had a non human creature so why werent they able to target that creature. Them stealing the entire stack and triggers, from my knowledge of the rules, that isnt how mutate works. I play alot of mutate in arena and that interaction was completely different from how every other interaction with mutate has worked.

EDIT:My bad, the below is wrong, due to me not realizing, that the cited rule (702.140a) talks about owner not controller (leaving this here for reference, though).

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Your mutate spell targeted a non-human creature under your control, as it cannot target a creature under another player's control. This is important, because when your opponent gained control of your mutate spell, the target became illegal. And when a mutate spell with an illegal target resolves, it resolves as a creature spell, the creature entering under the spell's controller's control. (See the rules below.)

702.140a Mutate appears on some creature cards. It represents a static ability that functions while
the spell with mutate is on the stack. “Mutate [cost]” means “You may pay [cost] rather than
pay this spell’s mana cost. If you do, it becomes a mutating creature spell and targets a non-
Human creature with the same owner as this spell.” Casting a spell using its mutate ability
follows the rules for paying alternative costs (see 601.2b and 601.2f–h).

702.140b As a mutating creature spell begins resolving, if its target is illegal, it ceases to be a
mutating creature spell and continues resolving as a creature spell and will be put onto the
battlefield under the control of the spell’s controller.