A fun little build I've been planning for a while. It might not be min-maxed, but its probably true to the Shadowlord and Assassin concept. This also requires the shadow-walker template to get into Shadowlord at 8, but you can wait until level 13 when Assassin spellcasting gets you dimension door.
Rogue 1/Fighter 1 (Hit and Run Fighter if you can manage it!), Rogue 2-3/Fighter 2/Assassin 2/Teflammar Shadowlord 6/Assassin 3-9
Take Two-Weapon Fighting and Dodge/Blind-Fight at level 1, Weapon Finesse at level 2 with your fighter bonus feat, dodge/blind fight at level 3 (the one you didn't get), mobility at level 5 from fighter bonus feat, and spring attack at six. Pick up craven, imp. two weapon fighting or other feats you enjoy at higher levels.
You have 7d6 sneak attack, and have a base Death Attack DC of 25. Assuming an Int of 16, +6 Int item, that'll be 31. Add in Bracers of Murder (Drow of the Underdark, +2 attack/damage against flat footed targets, +2 Death Attack DC, reroll 1s on Sneak Attack die), and an Assassin's Dagger sleeve blade (complete scoundrel hidden weapon), gives you a Death Attack DC of 34. You can boost it higher with a better intelligence, inherent bonus to intelligence for another two, or ability focus.
So, teleporting in behind someone and initiating a full attack you'll do 7d6+20 Sneak Attack Damage, +2 to hit and damage (they should be flat footed), and dex to damage from them being flat footed. Assuming a dex of 34 (18 start, +5 level, +5 inherent, +6 item), that'll be 8d6+29 plus your enhancement bonus on your weapon and other modifiers per hit.
Like I said, probably not the most optimized of Shadowlord builds, but its very rogueish!