Sneak attack/Critical help

Years ago I i remember a feat that allowed sneak attacks or crits against enemies that didnt have a discernable anatomy such as constructs or undead. I cant remember what book it was in, but all I remember is the artwork of a guy sneaking up behind a vampire and shoving a spear through his back and out of his chest. I know its a vague ask but I've been trying to find this and I feel like I'm hitting a Mandala Effect. It was something about a brutal sneak attack maybe. Ive searched through the Complete Series, Libris Mortis, and Book of Exalted Deeds and Im stumped.