The Fallen Prince and his Human Mate

The terrifying trio of demigods were a set for as long as they knew. Kaizen, his brother Tyson, and his sister Grace ruled a pack for nearly a hundred years. They are immortal, unable to even die... however, a rift is created when the oldest, Tyson, is named Standing King.
It is then that Grace leaves to find herself, in search of her mate, and Kaizen is forced to be Alpha of OpalMoon.
His leadership was not the problem. His skills as the New Crown Prince were taking on him, but not impossible... but it all crashed down on him with one lapse of jusdment.
The night he killed his fellow packmates on the Harvest Moon.
the One night the Moon Goddess, his grandmother, forbids killing, as it is the night a werewolf is most likely to receive their one, their soulmate.
.....and for a human no less....