For Death and Decay this is it...
Abilities\Spells\Undead\DeathandDecay\DeathandDecayTarget.mdx
Or it might be this one....
Abilities\Spells\Undead\DeathandDecay\DeathandDecayDamage.mdx
I don't remember which is which but try them both. A list comes with UMSWE I think (It's in my UMSWE files folder anyway...) so check it. I think you have to rename the extension mdl if you arent using it as a unit model though...
[edit] AHH! pigchow beat me to it
