This rogue build uses the Death Dealer's ability to penalize saving throws to deal a lot of powerful ongoing damage.
Race & Class Edit
Bugbear if you DM allow it (it isn't designed for PC).
Otherwise, Elves for their accuracy power.
The build is a Rogue with the Brutal Scoundrel class feature, which contributes to many ongoing damage powers. Multiclass to Fighter to get one more multi-target encounter power and encouner bonus to attack.
Ability Scores Edit
Str 16, Dex 16, Con 12, Int 10, Wis 12, Cha 8
Dex is determines your attack bonus, Str adds to sneak attack damage and to ongoing damage of some powers.
Stealth, Thievery, Acrobatics, Athletics, Insight, Perception.
Brutal Wound on herois and Bleeding Backstab on paragon are main targets.
Focus on ongoing daylies and multi-target encounters (since you can get into a mob of enemies, thanks to temp HP).
Wounding weapon (max level possible) to get down enemy saving throws (-2 from 11 level ability, and to -6 on 29 level weapon).
Paragon Path Edit
Death Dealer, of course.