The Tower of Endless Worlds by Jonathan Moeller

Give me a hundred guns, and I will conquer my world.
THOMAS WYCLIFFE just wants to finish his dissertation in peace and quiet. So when a man in a black robe appears in his closet, claiming to be the last of the Warlocks, Wycliffe figures it is a bad joke. But he soon realizes the last of the Warlocks can give him power beyond imagining. And all it will cost is his soul.
SIMON WESTER needs a job. Badly. So when a rich and powerful Senator offers him employment, he jumps at the chance. Sure, Simon expects to find some corruption, some shady deals. He doesn't expect to find black magic.
LIAM MASTERE is a Knight of the Sacred Blade, defender of the mortal races. But can swords stand against guns? As bullets and bombs destroy his kingdom, Liam must risk everything to save his homeland's one chance of salvation.
By daring the horrors of the TOWER OF ENDLESS WORLDS...