I think you mean something like this:
Triggering unit shackles some other unit, so when it shackles, every 1 second the damage done of immolation increases.
What i would try to do in 3 possible ways:
-Make a trigger that increases the level of immolation every 1 second(you will need to make more levels of immolation in object editor)
-Replacing old immolation with another immolation
-If the immolation damage increase only while shackles, make a periodic trigger that will damage nearby units, each time for more damage
Also make sure that triggering unit is channeling a spell(or if it has some buff) in case if it gets stunned or something. To stop the spell effects.