How to make projectile return?

How to make the projectile return like cryptfiend spider?
I want to do it but with a discblade projectile not spiderling.

When the unit with this projectile attacks another unit, just make a dummy unit with the same projectile with 1 damage attack the unit that attacked, or cast a spell on the attacking unit that does nothing with projectile art of the projectile.

You can use the Spider attack ability, it gives to the attack your desired effect.

EDIT: Damn, it keeps the spiderlings effect!! I'm afraid this effect must be triggered

Are you sure you can't change the spiderling effect on buff editor or something like that?

well, the missile goes and come back, but when it reach the target, it shows the spiderlings effect. Looks ugly.

I meant with buff editor but still, it turns out it is hardcoded and not buff based, you'll need triggers or to replace the spider attack model with the one you want

The problem is that this ability takes the unit missile, and it use it, except when it reach the target, changing it by the spiderlings. And the the missile return, using the unit's missile. I changed in the ability itself the missile art, but it doesn't work. It's a shame :(

Triggering this is the only good looking solution.

Make a custom unit based on the Crypt Fiend and change whatever you want. Trust me, I tried it once... Made a spearman out of it and the spear went back to him -_-'

Well i've tried the ability and i managed to remove the small spiderling with ability editor,i've made a new custom ability from the default spider ability called "disc return" and i set the "-spider capacity" to 0 and duration to 0.1 so it doesn't shown the spiderling.
But you can't modificate -missile speed (the missile is very slow by default) and -missile arc and the projectile is stuck on the ground with this ability (like any shokwave ability).So it sucks a bit.
I think the only way to have a good effect is tricky trigger as tidehunter said or pur hardcore jassing.