Desolation of the Sea (Sapphire City Series, #1)

In the shadowy depths of Atlantis, the once-majestic underwater kingdom is ruled by the tyrannical King Triton, whose cruelty extends even to his own sister, Ursula. Shunned and mistreated, Ursula endures unimaginable torment, culminating in a brutal act that leaves her both shattered and pregnant. As a twisted reminder of her suffering, she undergoes a horrific transformation into an octopus-like creature, her form reflecting the depths of her despair and anger.
Desperate for love and escape from her brother's oppressive rule, Ursula seeks refuge in the abyss Where she finds a cave to turn into her home. It is there she summons sea monster Cthulu to help exact revenge on her brother and get Ariel back.
However, the shadows of betrayal loom large. At sixteen, Ariel encounters her mother, leading to further heartache as she takes Triton's side. Ariel's heart brims with animosity as Ursula enacts her retaliation, only to find herself forsaken and deceived once more as Cthulu, captivated by Ariel, elopes with her.
Years pass, and Ursula amasses enough power to locate Ariel once more, only to face betrayal at the hands of her own daughter.