Lin Carter 
Tower at the Edge of Time 

It was Thane who crossed the abyss of the ages and stood on the very brink of Eternity…who found the secret of the Time Treasure… who saw, and laughed and came back sane from that place beyond the Universe where no man had ever been before him, and where none should ever come again.This was the ending. But the beginning was in the city of Zotheera on the planet Daikoon…A restless outworlder was Thane of the Two Swords. A man of blood and steel, a man of strange powers strangely acquired. The warrior Thane possessed a secret vital to the greedy Princes of the Planets. And it was on the planet Daikoon that the princes tried to buy Thane’s services…