Tosk is the hulking consequence of a murder.
1 year ago a Man murdered some witch's kin. As a consequence, Tosk was summoned into the sewers and underbelly of the city to haunt and stalk this murderer. Not to kill specifically but to remind him constantly and keep him afraid.
Tosk keeps the Bloody Murder weapon on him (a dagger constantly bloodstained) even showing it to the murderer from shadows and sewer grates when he can.

Solitary Troll, preying on the refuse of the city, and prowling the streets at night, Tosk is 8 ft tall, 500lbs doesn't always go unnoticed, but the Troll has grown a year in the sewers now, and has tasted human blood.