Bone Spirit-ish spell

I am trying to make a spell similar to Bone Spirit from Diablo, but slightly changed:

The hero casts the ability (non targeted) and a projectile will fly out in front of them chasing after the nearest enemy. If another enemy comes closer than the target (or target blinks further), the missile will chase them. I also want to make the missile be able to hit allies.
eg: Someone shoots the missle at you, you blink behind them, and the missile will go thru them to get you. When the missile hits its target, it deal damage to them (in that case, the caster )

Does anyone know how I could create a spell like this? Any help would be appreciated.

sounds like some kind of complex yet simple version of Fan of Knives, just replace the FoK caster and missle art and put some kind of projectile in, and if you want it to target allies (which is wierd) just change the targetting field

This would require triggering. Create a dummy unit and make it start following the closest enemy (periodic timer with movement) and check on each expiration for a closer target, in which case you switch.

Blu's way is the best.
Also, just so you know, every timer iteration check if the dummy is somewhere between 25-50 from any possible target, then issue the damage to that target.

This implies two groupings in the timer callback.
One grouping to check the nearest applicable target, and a second grouping if any applicable target is within range.

Ah k ty for help so far. But what if I shot two bone spirits, one after the other?

Make the spell use locals and such.
Depending on if you use GUI or jass the methods would be different.
In either case, your goal there would be multi-instanceability.