Beyond Fate by Lynn Hagen

When Damon is put into an ancient sleep by a malevolent vampire, he is awakened by the Ultionem using a key made from their combined blood. But after being infused with the blood of seven mighty beings, everything inside Damon goes horribly wrong. To make matters worse, his mate shows up to help battle the swarm of undead vampires, and contributes his powerful Unseelie blood, which ends up costing Damon dearly.

Once a noble commander of the king’s elite guardians, Casimir is branded a traitor and sentenced to death. He escapes the Unseelie realm by force, but spends the next two millenniums living in solitude, constantly on the move to avoid capture and execution. When a spell summons him to help fight an attack, he discovers his mate. Now, he must face the most difficult battle yet, because everything he cherishes most is at stake.