Teferi's puzzle box + notion thief

[Keyeto]

You got it right, except for this: if the thief dies, it's ability still resolves. It doesn't target him, so it does not become illegal. Once something hits the stack, it is independent of the source. Same happens if you draw an instant which destroys the puzzle box

Thank you for verifying sir! I actually had no idea abilities on the stack still resolved based on its target. I thought if a spell or ability is put on the stack on top of a previous (that destroys that permanent), that the previous spell on the stack fizzles.

Are you 100% on that, or anyone else want to chime in. I do believe you could be right, but this changes my understanding of how I thought the stack worked, the last 10 years of my life I have thought different lol..

I am absolutely certain. Check out the stickied post here in Rules, part of my FAQ for it is how abilities are independent of their sources once on the stack.

And if you won't take my word for it, here's the ruling! Bolded for convenience:

112.7a: Once activated or triggered, an ability exists on the stack independently of its source. Destruction or removal of the source after that time won't affect the ability. Note that some abilities cause a source to do something (for example, "Prodigal Pyromancer deals 1 damage to target creature or player") rather than the ability doing anything directly. In these cases, any activated or triggered ability that references information about the source because the effect needs to be divided checks that information when the ability is put onto the stack. Otherwise, it will check that information when it resolves. In both instances, if the source is no longer in the zone it's expected to be in at that time, its last known information is used. The source can still perform the action even though it no longer exists.

[imreallyagoodman]

Thank you for the clarification sir! Very helpful