Vengeful Cecaelia (2017)

From publisher blurb:

The half-human, half-octupus Cecaelia race is barely recognized alongside aquatic creatures like mermaids - and to be completely frank, mermaids much prefer it that way. Decades ago, the two races faced each other in a devastating war that boiled the seas and ultimately left the Cecaelia race almost extinct. Scholars have disputed since whether the mermaids were simply defending themselves or engaged in an organized genocide. What is completely certain is that the surviving Cecaelia are among the hardiest members of their race, and have almost exclusively sworn vengeance against the mermaid race.

The surviving Vengeful Cecaelia’s are a hardy race of spellcasting warriors, committed to utterly destroying their enemies. Experts in both physical and magical combat, a Cecaelia tends to wield weapons with its tentacles which keeps its hand free for spells. If one manages to gain the respect of a Cecaelian war party they can be valuable allies, but they will spare no quarter for mermaids. In fact, they’re suspected of using transformative magics to turn mermaids into humans - usually while they’re still trapped underwater.