A promising lead

Teleportations are generally impossible without the permission of Halaster Blackcloak. This would mean that the teleportation either was allowed by him or he could have an idea of how that could be done. Additionally if they are still located within the Undermountain, he would likely know where Sigurd is. Either way it is the best lead that they have. Darion is to meet with the mad mage in Skullport and see what he can find out