Damage Detection System isn't necessary. This spell should work like most triggered spells with a loop portion.
When the caster begins Immolation, run the periodic loop trigger, that for every X seconds of game time, picks all (friendly) units in an AoE around the Immolation caster and raises each one's HP.
Otherwise, you can add an Unholy Aura/Life Regeneration Aura ability to the caster when it begins Immolation, then remove it when the caster deactivates Immolation.
If you're looking for a quick and painless way to make triggered spells, visit Bribe's GUI Spell System.