If you were given a wand that could repeatedly cast a single spell, what would the spell be?

You've been given a gift of a wand that can only cast one kind of spell. You can cast the spell as many times as you like, but once you've chosen the spell, the wand can only cast that spell.

It also won't accept spells like 'create a wand that can cast any spell', or 'stop all war in the world'. It would be more the kind of spells you see in Harry Potter, like a healing spell, controlling spell, or even a killing spell.

If you were given a wand that could repeatedly cast a single spell, what would the spell be?
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