Intermediate triggering

Hello, I would like to trigger the following ability:

Casts spell, which fires "Missile art: ChimaeraAcidMissile" at speed 1000. When this hits, it deals damage based on intelligence and ability level when it hits. I then want a "Missile art: Death Tower" to fly back, from the damaged unit, and heal the caster for half what it damaged, again, when it hits.

I do not know how to code "on hit" or even how to code missiles.

I have based it off a spell, which allows the first missile to run well.

Here it is so far, and the timing is all off, as well as no second spawning missile.

Leech
Events
Unit - A unit Begins casting an ability
Conditions
(Ability being cast) Equal to Leech
Actions
Unit - Cause (Casting unit) to damage (Target unit of ability being cast), dealing (((Real((Intelligence of (Casting unit) (Include bonuses)))) / 10.00) x ((Real((Level of (Ability being cast) for (Casting unit)))) x 20.00)) damage of attack type Spells and damage type Normal
Unit - Cause (Casting unit) to damage (Casting unit), dealing (((Real((Intelligence of (Casting unit) (Include bonuses)))) / 10.00) x ((Real((Level of (Ability being cast) for (Casting unit)))) x -10.00)) damage of attack type Spells and damage type Normal
Special Effect - Create a special effect at (Position of (Casting unit)) using Abilities\Spells\Human\Heal\HealTarget.mdl

Do any of you know how to fix it? Thanks!

you will need a damage detection system for that. use the search function.
also this board has [ gui] tags, and if you paste gui text between them, it looks alot better ;)

You can also trigger it without damage detection as a projectile spell.

You make the base spell's effects invisible, and create a unit with the projectile model you want when the spell is cast. Move the unit towards the target at speed 1000, dostuff when it reaches the target, and repeat the procedure in reverse.

This would be likely easiest to do with xecollider. It would be the least work, anyway.