I ended up building a drow rogue|assassin hybrid that uses, of all things, Whirling Kusari-Gama as the go-to power. Venom Master lets you inflict vulnerable 5 poison until the end of the encounter with your Darkfire power, which triggers twice if you hit with both Whirling Kusari-Gama attacks. You recharge your Darkfire with an action point, and at level 16 you can target 3 enemies with it (Curseborn paragon path).
Other damage bonuses come from Skulker of Vhaeraun (both attacks), Carrion Crawler Brain Juice (both attacks), Venom Hand Killer (only the second attack) and Envenomed Ki Focus (only the second attack). The first two also give your attacks the poison keyword.
If you can't reach your enemies in one turn, you can use Poisonous Shuriken instead.
Your rogue half gives you a minor action and off-turn attack which both serve to trigger Sneak Attack, a back-up at-will power that gives a -4 attack penalty (rattling and Skulker of Vhaeraun L10 feature), and a daily power that maximizes your Assassin's Strike damage.
Your Hybrid Talent (Cunning Sneak) combined with Killer in the Crowd serves to give you a free OA per round if you manage to position yourself correctly, or otherwise hide from view if you're in trouble.
Malklyr, level 14
Drow, Assassin (Executioner)/Rogue, Curseborn
Guild Attacks (Hybrid) Option: Way of the Ninja (Hybrid)
Hybrid Assassin (Executioner) Option: Hybrid Executioner Fortitude
Hybrid Talent Option: Rogue Tactics (Hybrid)
Rogue Tactics (Hybrid) Option: Cunning Sneak (Hybrid)
Darkfire Option: Darkfire Wisdom
Born Under a Bad Sign (Born Under a Bad Sign Benefit)
Theme: Skulker of Vhaeraun
FINAL ABILITY SCORES
STR 14, CON 12, DEX 22, INT 9, WIS 22, CHA 11
STARTING ABILITY SCORES
STR 13, CON 11, DEX 16, INT 8, WIS 16, CHA 10
AC: 28 Fort: 23 Ref: 27 Will: 28
HP: 99 Surges: 7 Surge Value: 24
Skulker of Vhaeraun Utility: Skulker's Venom
Drow Racial Power: Darkfire
Assassin Attack: Whirling Kusari-Gama
Assassin Attack: Poisonous Shuriken
Assassin Attack: Assassin's Strike
: Swap Daily For Poison Recipe
: Swap Second Daily For Poison Recipe
Rogue Attack 1: Disheartening Strike
Rogue Utility 2: Tumble
Rogue Attack 3: Low Slash
Rogue Attack 3: Startling Offensive
Assassin Utility 6: Ghost of the Rooftops
Rogue Attack 9: Knockout
Assassin Utility 10: Walk Through Shadow
Curseborn Attack 11: Accursed Flames
Curseborn Utility 12: Drow Shadow Stride
Level 1: Ki Focus Expertise
Level 2: Venom Hand Master
Level 4: Nimble Blade
Level 6: Venom Hand Killer
Level 8: Battle Awareness
Level 10: Hybrid Talent
Level 11: Venom Master
Level 12: Killer in the Crowd
Level 14: Superior Will
Carrion Crawler Brain Juice
Envenomed Ki Focus +3
Magic Leather Armor +3
Amulet of Protection +3
====== End ======
Surprisingly, when you build this out as a Twin-Striking ranger instead, the damage difference isn't that big. The only unfortunate thing is that Whirling Kusari-Gama's second attack is dependant on hitting with the first attack, as opposed to Twin Strike's separate attacks. But apart from that, this build has minor and off-action attacks like the ranger, and has some cool rogue/assassin tricks (like hiding) to make up for the ranger's pure power.
The level 17 Rod of Feythorns inflicts vulnerable 10 poison (save ends) when you curse an enemy with Warlock's Curse. This is more reliable than Darkfire since you don't have to roll to hit and it's reusable, but a warlock|assassin misses out on a lot of rogue goodies (like the minor & off-action attacks) and has to deal with weapon/implement juggling issues. Overall I like the rogue version a bit better, especially since it's not dependent on getting a level 17 item. Still, it does make a good case for multiclassing warlock down the line for a once/encounter curse.