If your opponent hand bottomless shifting sands activated and the hightest attack power monster on the field was your spirit monster when you end your turn would the spirit monster be destroyed or returned to your hand?

Which one of these would happen and why?

Since the "Bottomless Shifting Sand" is your opponent's card effect, and the Spirit Monster is your card effect, it would be a simple matter of Priority in the End Phase.

Seeing as it is your turn, you have two options.

1) You can return the Spirit Monster to your hand.
2) You can pass priority to your opponent to resolve their "Bottomless Shifting Sand".

Lets say you use option 2...now, your opponent has two things they can do.

1) They can resolve "Bottomless Shifting Sand" (so that your Spirit Monster would be destroyed).
2) They can pass priority back to you, in which case you MUST return the Spirit Monster to your hand.

This is going off memory so some confirmation would help...

Envoy of toilet is correct. Any phase in which effects resolve, the turn player can first resolve his/her effects, or pass to the opponent for his/her resolution.