Sempiternal (Devil may Cry Book 3)

Book 3 to Immortals.

With the Demon Kings eliminated, the world is free from their horrible rule. Now, in order to keep the Humans safe, Cassi must travel across the world with the Nephilim to destroy the demons. The Hell Gate is unstable, causing the demons to travel to the real world from Limbo.

But in the midst of terror, evidence of a new threat rises, as they soon discover that someone else is feigning for the title of Demon King.

Even worse, Vergil and Dante have grown more taunt with each other after Vergil tried for more power.

It's up to Cassi to stop an all out World War III; but this time, with demons.