Base your ability on 'Unholy Aura'.
Set the life regeneration field to a negative value.
Set targets to 'enemy'.
Change the buffs attachments to whatever effect you so desire.
EDIT: Oh I see, you just want the Death and Decay effect.
As Scorpion suggested: Create a Dummy when the hero learns the ability. Use a periodic timer to order it to cast Death and Decay at the position of your hero.
Remember to clean up leaks, especially important with periodic events like these that can last an entire game, that's a lot of leakage.