Your Charisma score increases by 2, your Constitution score increase by 2, and your Wisdom score increases by 1.

Deep Thalassa Mermaids mature at a much slower rate than humans do, reaching adulthood around the age of 250. They live considerably longer, seemingly forever against the human civilizations that have sailed over them, but some have been seen to survive for about 900 years. Pod matriarchs rumor the longest living Deep Thalassa roamed for nearly 2,000 years.

Most Deep Thalassa mermaids are lawful, leaning towards evil, due to living in harsh environments. However, those that have been lost, drifting through the blackened trenches for far too long lose their sanity and are susceptible to turning Chaotic Evil. Engaging in territory disputes and attacking anything and everything that moves. 

You can speak, read, and write in the languages common and aquan. Additionally, you are able understand and write in primordial glyphs.

You are considered Humanoid. 

Deep Thalassa Mermaids vary widely in length, size, and build; from barely 3 feet to well over 16 feet long. Regardless of your position in that range, your size is either Small or Medium.

You have a walking speed of 25 feet and a swimming speed of 40 feet.

Accustomed to the bioluminescent life and volcanic rivers that glow faintly to guide you through the forever night soaked trenches, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can barely discern color in darkness, but they are muted and heavily saturated. In water, this extents up to 120 feet. 

You have disadvantage on attack rolls and Wisdom (Perception) checks that rely on sight when you, the target of the attack, or whatever you are trying to perceive is in direct sunlight.

You can breathe both air and in water. As long as your protective mucus layer and skin remain moist, you can survive several days out of the water, given the right conditions.

You have advantage on saving throws against being charmed, but magic can put you to sleep. However, you have advantage on saving throws against other non-magical agents that would put you asleep. Examples; non-magical/natural poisons or gases. 

You were born and fortunate to make it in the harshest of environments within the deepest of arctic trenches. Absolute darkness, predator filled, and freezing temperatures; all the makings of death for a normal mortal. Not only are you accustomed to the extreme deep aquatic pressure of the oceans, you ignore any drawbacks you'd receive when travelling in similar environments. Furthermore, you can withstand temperatures of extreme aquatic cold (0°F and below to -5°F), are able to survive through freezing temperatures on land, and have resistance to cold damage. Deeply-seated into your makings as a Deep Thalassa mermaid, you gain the innate proficiency in the skills; deception, perception, and survival. 

You need to sleep (8 hours), however it's extremely tough to do so in the darkness of the seas when you're alone and without a pod. Instead, you're able to go into a deep trance-like sleep, but remain semiconscious, for 4 hours a day.  After resting in this way, you gain the same benefit like you would from your normal 8 hours of sleep. However, you cannot use this feature again until you have completed a long rest of 8 hours.

You are no ordinary humanoid. Molded from high pressure literally and figuratively, your race has adapted and evolved to protect, hide, and weaponize every inch of yourself. Your transparent scales give you advantage on stealth checks when you are totally submerged in more than 10 feet of water and/ or in low visibility environments such as fog, and even in dim lighting or darkness. However, since you are a humanoid from the darkest and deepest parts of the arctic trenches, your appearance is uncanny and terrifying to most. Most social interactions and skill checks: performance, persuasion, and deception, against most humanoids are at disadvantage since you have a deep sea fear-inducing appearance. Although, you can turn the fear others have of you to your advantage. Any intimidation check or social interaction your DM needs of you where you need to use your appearance in your favor can be done so with advantage.

When you reach 3rd level, you can cast the Air Bubble spell on yourself or another creature you can see within 15 feet of you once with this trait. When you reach 5th level, you can cast the Invisibility spell on yourself once with this trait. You don't need material components for either spell, and you can't cast them while you're out of water and dry. If you still have your mucus layer to protect your fish scales, you are able to cast them with a 50% failure chance. You regain the ability to cast these spells with this trait when you finish a long rest. Intelligence is your spellcasting ability for these spells.