Darkness of Descent - Mages & Magic, #3

Tiamat goes back in time to Earth to stop demons from ever coming to a world of magic. Meanwhile, his allies must survive being trapped underwater by mermaids.

Peter - πύλη
An undead water mage, murdered for kidnapping a mermaid.
Orthros Όρθρος
Two-headed demon hound, leading demonic invasion
Lernaean Ύδρα
Nine-headed hydra demon, sibling to Tiamat

Orthros, a demon god from Earth, has promised the dragons they will rule the world, and thus, gained their alliance. In seventy-two hours, an armada of demons will arrive and assist the dragons in enslaving all goblins, humans, and elves. Tiamat goes to Earth to stop the armada, but time-manipulating gods intervene and refuse to let him return until he corrects a mistake he made a hundred years ago when he first left Earth.
Rojo Siete is in charge of rescuing Mage Leslie, Elf Ruven, and a nest of gold eggs during a three-day cease fire. The mermaids have taken them and command all creatures in the sea. The goblins are scheming to rescue them first, for darker reasons than others know.

This story is available in two adaptations. The original, Mages & Mates, is a romance filled with encounters involving dragon shape-shifters. The alternative adaptation, Mages & Magic, is censored and edited to be appropriate for all ages. It has the same main story, but some aspects and themes are changed to keep it family friendly.

So, pick the version that fits your taste and enjoy an epic adventure with an unlikely group of anti-heroes.