Cerebus

Cerebus by J.J.F. Musgrave:

Cerebus follows Ellie Carson, a young woman drawn into a war she never asked for, when the myths of the ancient world erupt back into reality. Her unlikely ally is Cerebus - not the demon of legend, but a scarred, loyal three-headed golden retriever, each head with its own mind, instincts, and heart.

Armed with the Blade of Bellerophon, Ellie and Cerebus are thrust into a desperate struggle against the Chimera, a monstrous force of fire, venom, and destruction that lays waste to cities and civilizations.

As governments falter and militaries fail, Ellie and Cerebus forge a bond of trust, sacrifice, and love, learning to fight as one. Together, they must navigate labyrinths of myth, global chaos, and their own fears to protect a fragile world standing at the edge of apocalypse.

The story builds from training and mistrust, to alliance and survival, and finally to an epic showdown in the ruins of Rome-where a girl and her dog face the end of history itself.

But victory is not the end... because the gates to other myths are beginning to open.