One Fatal Tree. The Carolingian, #3

Richard Oort, Paladin and a warrior capable of wielding an ancient and mysterious weapon, has sworn to defend the world from invasion by the horrific Old Ones. If he can. Alone and overwhelmed after a series of catastrophic defeats, Richard must still find a way to save the world, no matter the cost to himself. However, if he pays the ultimate price and dies without a new Paladin to replace him, humanity will plummet into the next Dark Age, bereft of reason and hope.
At the darkest moment, and against all odds, a new Paladin arises. There's just one problem: Mosi is only nine years old. A Navajo girl who lost her entire family to the machinations of the Old Ones, Mosi is alone in the world. To keep her safe, Richard - a single man with zero parental instincts, and who is in constant danger from his work as both a detective and a Paladin - must become her guardian.
When Richard is betrayed by his own side, he flees with Mosi, and the man he secretly loves, Damon Weber. They, together with a group of scientists, take refuge with a secret society in Turkey, safe at last. But tragedy strikes when Richard loses his sword, the weapon that gives a Paladin their power, and time is running out. Despite the increasingly dire circumstances, Richard receives help from the most unlikely of sources.
The man Richard has adored in secret for years unexpectedly declares his feelings for Richard. With that love, and the faith of a little girl who's become like a daughter to him, Richard finally discovers the strength to do what he must.