Even the enemies on those super and ultra nightmare difficulty mods shouldn't be able to stand up to this...a "Seriously, how am I supposed to get out of this mess?!" kind of item :)
It's the Starfang 2H sword from Warden's Keep DLC, with quite a few surprises that'll leave your enemies helpless as you slaughter mass hoardes...
A word of caution: This sword MIGHT take the fun out of playing the adventure...just saying.
Note: You WILL need the Vaddi mod and dev console enabled to get this.
http://social.bioware.com/forum/1/topic/307/index/6587675
Install- Place ERF file in your override.
Obtain- Open console -> "runscript vaddin Starfang"
Uninstall- Remove ERF from override
For those curious about the stats...
4 rune slots
48 Phys Damage (79 DPS)
+1,544 Phys Damage
+77% Phys Damage
Attacks Ignore Enemy Armor