AKA, a Mawile and Mareanie fusion. I've been sitting on this one for ages, finally got it done.
Name: Sirenity, from Siren and Serenity.
Height: 4'6
Weight: 77Ibs
Type: Poison/Fairy
Ability: Merciless, Strong Jaw, Rivalry (hidden)
Sometimes, when diving into the deep depths below, one may catch a fleeting glimpse of this Pokemon's dark, elegant appearance. People and Pokemon alike who follow the alluring creature to its underwater den are said to never see the surface again.
After incapacitating its prey with poison pins fired from the tip of its head, Sirenity uses its powerful jaws on the sides to tear into its incapacitated meal.
Sirenity are known to have fierce rivalries with one another. If one returns home to find that another Sirenity has taken residence in its underwater cave, the two will battle violently until one of them leaves. It has been shown to take care of Mareanie, however.