not muiahh.
delete the effect of it then
Parasite
Events
Unit - a unit casts a spell
Conditions
Spell being cast equal to Parasite
Actions
Set ParasitedUnit<gen> targeted unit
Set Parasiter<gen> triggering unit
Trigger - run Pain
Pain
Events
Conditions
Actions
Unit - order ParasitedUnit<gen> to attack ParasitedUnit<gen> dealing 1 hp damage of type Fire
Wait 1 seconds
If ParasitedUnit dies then do then actions actions else else actions
Then actions
Unit create 1 Your-Naga-unit... x levels of parasiter<gen>
Skip remaining actions
Else actions
Trigger - run Pain
hope it works..