Blackskate (CR 4)

Large Undead (Aquatic)
Alignment: Always neutral evil
Initiative: +3 (Dex); Senses: darkvision 60 ft., scent, Listen +4, and Spot +4

AC: 18 (+3 Dex, +6 natural, -1 size), touch 12, flat-footed 15
Hit Dice: 4d12 (26 hp); DR: 5/slashing
Fort +1, Ref +4, Will +5
Speed: Swim 40 ft.
Space: 10 ft./5 ft.
Base Attack +2; Grapple +11
Attack: Stinger +6 melee
Full Attack: Stinger +6 melee and bite +1 melee
Damage: Stinger 1d6+2 plus poison, bite 1d8+5
Special Attacks/Actions: Blood rage, poison
Abilities: Str 20, Dex 17, Con -, Int 12, Wis 12, Cha 16
Special Qualities: Blood tracker, scent, undead traits
Feats: Stealthy; Swim-By Attack
Skills: Escape Artist +5, Hide +8 (+12 on seafloor), Listen +4, Move Silently +12, Search +4, Spot +4, Survival +8, and Swim +16
Advancement: 5-12 HD (Large)
Climate/Terrain: Any aquatic
Organization: Solitary or pair
Treasure: None

Source: Stormwrack

The thing before you undulates in the currents, moving wing-like projections at its sides like a pallid manta ray. Its body is on amalgamation of tattered flesh. Its is face made up of shattered pieces of bone roughly pulled together into a mismatched puzzle, as is the tail that now arcs scorpionlike over its back.

A blackskate is made up of cast-off bits of flesh, bone, scale, and cartilage that settle to the ocean floor. Somehow this organic detritus mixes with the dark currents of the blackwater trenches and a terrible undead creature is created. A blackskate's body is difficult to damage: the best way to destroy one is to hack it to pieces, which isn't an easy task in the water.

A blackskate seeks out living, warm-blooded creatures. Though it does not consume blood the way vampires and their ilk do, warm blood drives a blackskate into what merfolk call the blood rage - a state in which the creature becomes even more formidable.

A blackskate is capable of inexorably tracking anyone whose blood it has scented, so long as it and its prey are in the same ocean.

Aquatic Subtype

Creatures with the aquatic subtype always have swim speeds and thus can move in water without making Swim checks. An aquatic creature can breathe underwater. It cannot also breathe air unless it has the amphibious special quality.