Dawn of Night. The Erevis Cale Trilogy

Tensions escalate between Erevis Cale and his rival-slash-accomplice, Drasek Riven, in a fierce battle of wills Lost on the mysterious Plane of Shadow, with no way home in sight, Erevis Cale and his band of comrades recover from their recent battle with the fearsome Slaadi-a species of monstrous and frog-like creatures. Meanwhile, Drasek Riven grows more calculating as Cale's powers increase, and the rivals are locked together in a complicated power struggle-sometimes as enemies and sometimes as accomplices.
As the duo makes their way to the infamous city of Skull Port and to the chaotic Skulls, a band of wizards tasked with patrolling the city, they will push their fraugt alliance to its furthest limits.