The Enduring Flame Trilogy

A second trilogy written by Mercedes Lackey and James Mallory, The Enduring Flame Trilogy is set a thousand years after the events of The Obsidian Trilogy. Kellen and Idalia are worshiped as religious figures, and High Magic is long gone. However, Tiercel Rolfot and his friend Harrier Gillain embark on the adventure of their lives when Tiercel discovers he is a High Mage.

Meanwhile, in a far-off desert, a Wildmage named Bisochim comes to believe that the world is out of balance without darkness. He formulates plans to bring back the dark, against the wishes of his bonded dragon, Saravasse.

Key figures:

Tiercel Rolfort: A member of Armethalieh's minor nobility, and a prospective student at Armethalieh University, he finds a book on High Magic and casts a spell that mysteriously causes him to fall seriously ill, driving the two boys to head out of Armethalieh to find a Wildmage to cure him. He later discovers that he is the first High Mage in almost a millennium, and bonds with Ancaladar after the elderly Jermayan dies, in order to obtain a source of energy for his spell-casting.
Harrier Gillain: Tiercel's friend since childhood and youngest son of the Harbormaster of Armethalieh, he accompanies Tiercel to seek a Wildmage to cure Tiercel, as well as to take care of Tiercel on the way. He later discovers that he is a Knight-Mage, and learns swordplay under Macenor Telchi.
Ancaladar: Having bonded with Jermayan for a millenium, Ancaladar joins Tiercel and Harrier after Jermayan dies , Elven King Sandalon having performed a Great Spell to transfer his bond to Tiercel. He acts as a mentor to Tiercel, training him in the use of the High Magic throughout their journey.
Bisochim: A Wildmage who lives in the Isvai, Bisochim bonds with a dragon named Saravasse after she is injured and he nurses her back to health. He believes that the Wild Magic is out of balance, with too much Light, and intends to summon an Elemental Spirit of Darkness, Ahairan, in order to bring Darkness back into the world to counterbalance the Light. It is Tiercel and Harrier's goal to stop him from doing so.
Kareta: A unicorn who brings Harrier's Books to him, and joins the party for a short while. Constantly snarky, and getting on Harrier's nerves, she nevertheless aids them through the Elven lands (by purifying water sources, for instance). Revealed to be Shalkan's daughter.
Macenor Telchi: A Selkan warrior whom the group rescues from near-death, he acts as The Obi-Wan to Harrier, teaching him the Selkan dual-sword form of swordplay. Later dies during the siege of Tarnatha'Iteru.
Shaiara: Leader of the Nalzindar tribe of the Isvai, she leads them to a long-abandoned city in order to escape the other Isvaieni's purge of the Iteru cities. Later, she becomes Harrier's love interest.

Tropes used in The Enduring Flame Trilogy include:

Apocalypse How: Regional: By the end, a large part of the Isvai desert has been environmentally wrecked, with oases, plant-life and wildlife absent save in certain areas, and the String of Pearls has been completely depopulated, with some cities completely demolished. The Isvaieni themselves have been reduced in number by around 2/3rds through attrition, with a number of tribes completely wiped out. On the other hand, the rest of the world has been left untouched.
The Atoner: After learning how he had been deceived by Ahairan, Bisochim becomes this.
Childhood Friends: Tiercel and Harrier.
Hell Hound: The Black Dogs created by Ahairan seem to fill this trope.
Heroic BSOD: Tiercel, after Ancaladar's disappearance.
Heroic Sacrifice: The Firecrown informs Tiercel that one will be performed before Ahairan is reimprisoned. Tiercel spends an entire book preparing himself to do it. It turns out to be Bisochim.
Put on a Bus: Ancaladar, at the beginning of the third book. He doesn't return until the very end.
Recursive Reality: Tiercel proposes that the worlds are each nested inside the other like a puzzle box. When "lined up" properly, something can move from one world to the other.
The Unpronounceable: In the final book, we find a tehukohiakhazarishtial. This is a hollow mountain filled with molten rock, or as most English-speakers would say, a volcano.
Well-Intentioned Extremist: Bisochim considers himself this.