Chain Damage/Heal Spell Trigger

[Quauhtli]

I am making a spell that will take some triggers to function well. Thing is I don't know where to start.
It works like Chain Lightning and also like Healing Wave.
The spell can be cast on anyone- both friend and foe- and has a 50% chance to do damage and a 50% chance to heal. However like Chain Lightning and Healing wave it bounces to the next unit and keeps on doing less damage/healing and still on every new bounce has the 50% chance.
I don't really know how to trigger the second and third and so on bounce.
Anyone have a clue?

 

[N2o)]

you can try looking at this and adapt it to your needs.