If you could invent your own spell in the world of Harry Potter, or alternatively a crossover, what would you like it to be?

One of mine would be an armour spell that could protect the caster, even if temporarily, from the 3 unforgivable curses, in particular Avada Kedavra, giving you an edge over dark magic users who don't hesitate to launch this spell repeatedly in combat. It would be made of energy so it wouldn't weigh down the caster, and would allow freedom of movement whilst covering vital areas of the body. Basically a more advanced shield spell, so only available to dedicated and disciplined practitioners of defense against the dark arts.

Oh... Well there is this one spell I had my mind on...

It's a spell of which causes your wand to break into a million of smaller-then-microscope-can-see and causing it to go into your fingers, enabling you to use magic with JUST your fingers. (Ministry of Magic keeps tabs on those who try it. Class level one higher then Animagus.)

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I'd create something called "Vocabo" meaning "Summon". Unlike "Accio" -- meaning "I summon" -- it would straight up materialize whatever the caster wants.