Sorry if that has been asked before (read lots of FAQs and used search but couldn't find anything *accurate* ) :
How does exactly "decay" work ? Is it anything more than a (relatively slow) damage over time spell ?
The decay and poison effects have not changed from dom 1, when the battle is over the remaining poison damage is dealt to poisoned units. Non regenerating decayed units die, units that regenerate sufficent amounts survive. There is MR on decay.