Mind over matter

[Deathstorm]

If I have {mind over matter} on the field with a untapped {arbiter of the ideal} and use the ability of mind over matter to tap it. then my opponent activates a {royal assassin} in response to the royal I activate mind over matter again to untap my arbiter does my arbiter die or does the assassins ability fizzle?

[Nymuera]

Creature lives. Last thing on the stack, untap, goes off first. Creature untaps. Next on stack, destroy target tapped creature, no valid target, effect fizzles.

[Destore117]

Would he have to target his assassin with the ability as that is the only valid target?

[Giggle the Draco Genius]

Once a target has been chosen the target can not be changed unless another effect can change it.  SInce it is not longer a valid target as it is untapped, the ability just fizzles, or is countered upon resolution.

[Sardok]

Actually when {Royal Assassin} is activated {Mind over Matter} is still on the stack and hasn't resolved yet. This means your {Arbiter of the Ideal} is still untapped.

Therefore, {Royal Assassin} can't activate as there is no tapped creature yet.

[cltrn81]

Actually your creatures dies because of last in first out.
1. Your creature is tapped.
2. You activate {mind over matter}
3. While the untap effect is on the stack he responds with {royal assassin}
4. His assassin resolves first and your creature is still tapped so it dies.
5. Then the untap effect resolves and fizzles.

You could keep your creature but you would have to do another {mind over matter} effect in response to his {royal assassin}

Oh nvmind I just realized you said your creature was untapped.  Well this would happen if you tried untapping 😏

[Pleeb]

Actually when {Royal Assassin} is activated {Mind over Matter} is still on the stack and hasn't resolved yet. This means your {Arbiter of the Ideal} is still untapped.

Therefore, {Royal Assassin} can't activate as there is no tapped creature yet.

Let's assume that the first mind over matter resolved before the assassin was tapped to kill him.