My thoughts:
It’s wonky.
I understand that the idea of making a carrier ability instead of just “Weapon Doom,” gives it a more unique feeling, and I suspect that might be why it functions like this. It’s a Rogue capstone that doesn’t feel like Eviscerate.
That being said....I wish it was just Weapon Doom. As is, it’s far too reliant on getting through Armor. You could use a Vorpal Serious with some backstabs to do something like 20-30 Doom from behind, I guess, but even that’s no guarantee.
In its current state, it’s pretty much only good coupled with a high damage skill, which will basically be Assassinates or Eviscerates (I guess Slays is possible for some Scout builds, if scouts choose to invest in Slays, but I suspect that most won’t).
I could also see it used as a stand-alone skill for groups who like to use Shatter on enemy armor, or on NPCs who still rely on huge body counts.
That being said....5 is probably too high in its current form. I’d pay 5 for “Weapon Doom.” I don’t think I’d buy this skill more than once as a Rogue.