I think it'd make more sense to be instant-cast, and have a low-accuracy Doom effect that may or may not proc.
If it does proc, it should usually just wear off before the timer (maybe 3 tics) expires. The spell would be cast for damage, and it would be called Darkness or something and not just straight up Death or Doom. The added effect would just make it more interesting, not necessarily add much power to it.
Every now and then, the timer would go all the way down and result in a KO.
