Nice idea!
Its a passive spell and I call it ''Natures Blessing''
Its a passive spell for a whole race and for units only, units are trainable in the buildings too (this is just an idea for where should I use the spell)
Ok now for the spell
- Whenever the unit with a ''Natures Blessing'' passive ability, drops his health to 25% he will be healed by 1% of his max health for 10 seconds.
-The ability also has cooldown of 1 minute, so the passive ability wont work even if the units health drop to 25%.
Could you make it vJass? But if it wont be easy for you GUI will do.
Edit: Wietlol told me these, maybe this might help.
Use a DDS.
When a unit takes damage (only way how you should lower their HP), check if that unit has ability (...).
If so, check if the health is lower than 0.25 X max health.
Then create the effect... you can use my EOT System to do that... you should know how to by now
Also create a timer and save that timer in a hashtable under the key of the unit and 0.
(You can get the key of the unit by a custom script "Custom Script: set udg_MyIntegerVariable = GetHandleId(udg_MyUnitVariable)".)
before you start the effect, check if that timer exist.
When that timer expires, you will destroy it.
(You need JASS for that last one.)