Sunken Herald.

Your innate magic comes from strange undersea powers, spawned from creatures of the deep that lurk in furtive darkness. Krakens, merrow, sea hags and other marine horrors often abduct thralls from the surface, controlling them with hypnotic magic before discarding them and leaving them forever changed. You have a connection to one of these thralls—or could even be one of these thralls yourself—as the descendant of a deep scion, a child stolen by a sea hag, or someone anointed as an acolyte by a circle of kraken priests. Your inextricable connection to the depths warps your body and soul into strange, alien forms that terrify your terrestrial allies. Above all, the sea lures your mind, bringing you both calm and deep foreboding near the open waters.
Regardless of the source of your power, you fill those that lay eyes upon you with dread. The terrors of the depths and the horrors that live on the cold, black seafloor weave their way into your magic and your very body. Despite this, powerful Sunken Heralds tend to become the leaders of far-reaching cults and powerful secret societies.

Level 1 Features:

Between Worlds.

Your foul ancestry grants you abilities some consider unnatural. At 1st level, you learn Deep Speech, can breathe both air and water, and have a swim speed equal to your walking speed. Additionally, you can walk across the surface of liquid water as if it were harmless solid ground.

Malleable Flesh.

At 1st level, you can cause the magic that flows through your veins to shift and change your body to suit your needs. When you cast a spell of 1st level or higher, you can modify your body in one of the ways listed below. If you cast a spell of 2nd level or higher, you can choose one additional feature for each slot level above 1st.
This change lasts for 1 minute, until you choose to use this feature again, or until you end it as a bonus action on your turn.
Elongated Limbs.
Your arms and legs snap and stretch unnaturally, tripling in length. Your reach is increased to 10 feet on your turns, and your speed increases by 10 feet.
Freakish Graft.
The flesh of your hand swells and grows around one object that you're holding, mingling them together. You can't be disarmed of the item or drop it. If you are holding a structure or object too large for you to move, you can't be moved more than 5 feet away from that object.
Mucosal Escape.
Thick, foul slime coats your skin. You have advantage on checks to avoid being grappled, and opportunity attacks against you are made at disadvantage.
Net of Veins.
Your torso bursts into a web of blood vessels that anchors itself on structures and objects that aren't being worn or carried within 60 feet of you. Until this effect ends, you have advantage on checks and saves to avoid being knocked prone or moved unwillingly, and can use a bonus action to pull a Small or smaller object within range that isn't being worn or carried into your space. You can use a bonus action to focus your veins on a destination you choose within 60 feet of you. When you do so, the veins can fasten themselves securely to an object at that point, unfasten themselves, or spread evenly around you again.
Predator's Claws.
Your nails lengthen and sharpen into thin, translucent spindles, which you can use to make unarmed strikes. If you hit with them, you deal magical slashing damage equal to 1d8 + your Strength modifier.
Salt and Sunder.
Corrosive brine drips from your hands, which become pitted and blackened. Your attacks and spells deal double damage against objects.

Level 6 Feature:

Weird Reconstruction.

At 6th level, the bizarre biology that gave you life allows you to reform your body in horrific ways. When you take damage, you can use your reaction to warp your flesh back into shape. You gain temporary hit points equal to your sorcerer level, and if the source of damage was a creature, it must make a Wisdom saving throw against your sorcerer spell save DC. On a failed save, the target is frightened of you until the end of your next turn.
You can use this feature a number of times equal to your Charisma modifier (a minimum of once). You regain all expended uses when you finish a long rest.

Level 14 Feature:

Monstrous Mass.

At 14th level, your contorting, inhuman body terrifies and disorients your enemies. When you activate Malleable Flesh by casting a spell of 6th level or higher, each creature of your choice that is within 60 feet of you must make a Wisdom saving throw against your sorcerer spell save DC. On a failed save, a target is frightened of you until your transformation ends. While transformed in this way, frightened creatures have disadvantage on saving throws against your spells.

Level 18 Feature:

Pelagic Nightmare.

Starting at 18th level, you can spend 5 sorcery points as an action to temporarily manifest images of an ancient, sunken city around yourself for 1 minute. All light within 120 feet of you is reduced to dim light, and creatures of your choice must use their swimming speed to move through the area, rather than their walking speed, and can't benefit from cover. When you create this effect, you can designate any number of creatures you can see to be unaffected by the visions of ocean depths. When a designated creature casts a spell or makes an attack while in the area, it can treat any point within 120 feet of you as the origin of the effect, as though it were standing in that space.