What skill is this?

ESOUser0x00

Was doing a dungeon in the gold coast, PVE and I saw this guy, think he was a nightblade level 20 or so, sneaks up to every enemy and stabs them from behind, it was an insta kill for each one of them

Seraphayel

Blade of Woe I guess.

http://elderscrolls.wikia.com/wiki/Blade_of_Woe_(Online)

JasonSilverSpring

I thought the blade of woe could only be used against non-hostile NPCs.

runagate

You can use it on mobs. You get less xp for it, though. And obviously it won't work on bosses.

There's even a Set which procs from using it:

http://elderscrollsonline.wiki.fextralife.com/Sithis'+Touch+Set