At the cost of a morbid ritual, an eldrathul is granted an edge above the rest of the world.

Ritual of Sacrificial Control

The eldrathul has always carried out a ritual sacrifice: One life for each day of power they are granted. If this ritual is not followed, they lose control of their character completely. The DM takes control of the character. With regard for personal safety and complete disregard of morals, the character consumes the most convenient brain and returns control back to the player.

'Consume Brain' action:

If a target is sentient, grappled, below 12 hit points, and within 10 ft:
'Consume Brain' may be used to kill the target without an attack roll (with a saving throw) and commune with the Elder. This process takes a full ten minutes, and the target cannot break themselves from the grapple once the process has started. The target may attempt to make noise once per minute.

In the event that a target is broken free from the grapple, the target begins the process of becoming an eldrathul.

If the target is above 12 hit points, the target takes 1d8 damage per hit until they are below 12 hit-points.

The eldrathul will always hold a small part of themselves that distrusts what they are due to the memory gap that covers up what this ability may look like...

An example of what that might be;
The eldrathul's face literally splits into 6 different sections, revealing a beak in the center of their skull. The eldrathul then drills into the Targets skull, and promptly begins the process of consuming the targets brain.

Timeless Existence

Eldrathul enjoy an extended lifespan, often living for several millennia. This longevity allows them to accumulate vast stores of knowledge and arcane prowess.

Lost In Time

Once an eldrathul consumes its first mind, the Great Old Ones grant either their muscle memories or better compatibility with the infection. Either way, they also grant the eldrathul their thicker rubbery skin.
+6 to CON or DEX,
Resistance to piercing and slashing damage.

Secluded Enclaves

Eldrathul tend to establish isolated and secretive communities, often located near ancient, submerged ruins or coastal regions touched by the eerie presence of the deep sea.

Amphibious Nature

Due to their association with the sea, eldrathul have a natural affinity for aquatic environments. They are expert swimmers and can breathe both in air and underwater. Swim speed is equal to walking speed.

Insomniac

Need for sleep is negated

If an exhaustion level is gained, sleep can still help.

Dark Vision

Eldrathul live in the shadows, and have adapted accordingly. 60 ft

Tentacles

Eldrathul possess subtle, retractable tentacles that can extend from their wrists, offering unique abilities beyond having a 15 foot reach to interact with objects.

use 'Tentacle Whip' to deal 1d4+1 damage.
Once per short rest, you can use 'Tentacle Grapple' to attempt to grapple a target within 10 feet as a bonus action