Octanthropy

You have been cursed by an otherworldly being, taking on a form of a creature of the deep. 

Shapeshift: as an action you can sprout tentacles, up to a total limb count of 8, They use Dex instead of strength. As a bonus action, you can use two of them to make an attack, but the weapon must be light. Your face also grows tentacles and your eyes turn a pitch black. If you have hair, it all falls away, growing back after you change back into human form. If you have scales, they smooth out.

While in this form:

• You know the detect thoughts spell and can cast it once per short rest
• Gain a swim speed of 30ft.
• You have disadvantage on all persuasion, deception based checks
• You have sunlight sensitivity
• You have advantage on stealth checks if the moon is out, or if you are in any amount of water
• You have resistance to Psychic damage
• You have blindsight out to 20ft and darkvision out to 100ft

While not in that form

• While you are conscious, You must consume at least a gallon of water, or submerge yourself in water, every two hours, or take 1d6 damage every hour past two you haven't done either of those things.
• Your skin is clammy and is damp to the touch
• You must make a DC8 deception check when talking to anyone who is aligned with a deity, otherwise they get a very wrong feeling about you

At all times while this curse is in place:

• The spell detect good and evil will reveal you (evil), but does not change your alignment
• You have darkvision out to 60ft
• If the weather is hot or dry, make a con save every hour or take 1d6 damage
• You have resistance to Necrotic damage
• You are vulnerable to radiant damage
• You can understand and speak Abyssal