This is an omnibus reprint of Diane Duane's first two titles in this series, The Door into Fire and The Door into Shadow.
In The Door into Fire, Herewiss, Prince of the Brightwood, is the only man in centuries to possess the Power of the Flame, but he cannot control it. Even though he cannot control the Power of the Flame, he does have a talent for sorcery and with the help of Sunspark, a fire elemental, he is able to free his lover Freelorn, the Prince of Arlen. Now Herewiss must make a choice. Does he go with his lover to aide him in his fight to regain his kingdom or does he follow Sunspark to an ancient castle where there are 'doors' that lead to other worlds? Places that he can go to earn to use the Power of the Flame. Dare he walk through the door into fire?
In The Door into Shadow, we are at a time that the royal magics are failing. The Reaver armies are massing and with them is the Shadow-destroyer off all that is good in the Goddess's creation. Between them and humanity stand a few brave souls: Herewiss, the first man in decades to have the Power of the Flame; Freelorn, the prince of Arlen; Sunspark, a fire elemental; Eftgan, the warrior queen of Darthen; and Segnbora, sorceress and swordswoman who speaks with the tongue of dragons. Can they overcome their own past and differences to unite in this fight, or do they fall prey to doubt and distrust which leads to the door into shadow?
I read and loved Door into Fire, but haven't read Door into Shadow yet. There's also a 3rd, Door into Sunset. Also, a warning: (well done) male-male pairing.