Question the Underbelly Lowlifes in the Underbelly to find more information about Raest Magespear's location.
So my brother has gone off to perform some ritual with these cultists? That explains why he left, but still not to where... Wait! This letter mentions some of these "Nethersworn" cultists seeking reagents in the Underbelly. Perhaps we still may be able to find them if we hurry!