Venenum

Venenum Assassins specialize in poisoning people (their name is "venom" with extra "en"). Games Workshop doesn't seem to care much about them (probably because "I poisoned the well and caused your entire army to get sick and die" doesn't translate well to game rules).

Challenge accepted

Couldn't be that hard

WS 5, BS 5, S 3, T 5, W 2, I 6, A 2, Ld 8, Sv 4+ (200 points?)

Wargear

Venom Grenade, Range 12", S2, AP4, Poisoned 2+, Blast, Assault 1 (can be upgraded to Bane Grenades (large blast and AP3) for 20 points)
Venom Spray, Range ---, S2, AP4, Poisoned 2+, Template, Assault 2, twin-linked (Can be upgraded to Bane Blaster (torrent) for 15 points)
Venom Injector, Range ---, S-, AP3, Poisoned 2+, Melee, Mastercrafted (Can be upgraded to Bane Gauntlet (Fleshbane) for 20 points)
Synskin (Armour save of 4+, Inv save of 5++, feel no pain (5+)

Special Rules

Infiltrate

Scout
Hit and Run
Immune (Can reroll failed saves against weapons with the poisoned special rule)
Night Vision
Stealth

There ya go that wasn't so hard now, was it?