Nagpa/Skeksis for Call of Cthulhu 5e

Nagpa from the Creature Catalog (DMR2). Definitely not a Skeksis.

The Call of Cthulhu setting is already infested with ancient humanoids, so here's another one to help fill in some of those millions of years of secret history.

You can go full Dark Crystal with them if you like, but I'm pitching them as another form of 'immortal inhuman sorcerer' and basing them on the D&D Nagpa (which appeared the year after the film was released).

Nagpa, Withered Scholar of the Wastes.

With a vulture-like head and features of both avians, reptiles and mammals, the Nagpa are remnants of an elder humanoid species (though when they flourished in history/prehistory or if they are even native to this dimension/world is unknown).

Hunched over, sometimes crawling on all fours, a Nagpa appears shorter than it actually is, and multiple layers of clothing and ornamentation disguise its emaciated, spindly-limbed (though tough and sinewy) body. Clothing and equipment are usually of conventional materials, albeit remarkably preserved considering the decrepit state and obvious age.

Cursed with immortality, a Nagpa crumbles to dust when slain by violence and then spontaneously reincarnates -fully formed- from a leathery egg sac in some lonely secret place half a world away.. From there, it must begin its miserable existence again, seeking out sources of ancient science and sorcery in an attempt to solve the endless, aching cycle of being.

Nagpa are usually solitary, as they cannot bear the presence of their own kind for long, but do maintain relations with each other, via magical or technological means. They cooperate for preservation, trade in servitors and artefacts, and the advancement of their researches, but each would betray another in a dusty heartbeat for the chance of true death.

While a Nagpa cannot actually 'die' of fatigue or starvation, they feel the effects and can suffer permanent damage within their current incarnation, so they eat, drink and rest as required by their own metabolism and any modifications they have applied.

The Nagpa have a materialistic theological objection to the deities of the Mythos and don't believe in them.

STR 2d6+6 (13)     CON 2d6+12 (19)     SIZ 3d6 (10.5)     INT 3d6+6 (16.5)     POW d6+12 (15.5)

DEX 3d6 (10.5)                                        

Hit Points 14-15    Damage Bonus nil     Move 8

Weapons:

• Skinny claw 30%, d3
• Staff 50%, d6 (Parry 50%, 20 hp)
• Sword 35%, d8 (Parry 70%, 10 hp)
• A Nagpa may also have access to other armaments, including energy weapons, and could have converted those of other monsters to its own use. They will understand contemporary firearms and explosives readily enough, but are unlikely to have previously encountered such human instruments.

Armour: none naturally, but layers of clothing equivalent to d4 points and may wear additional protection.

Skills: 

• At least INT x 2 + any base chance in all appropriate Knowledge skills. Nagpa will specialise in several areas - they have the time and the need to do so.
• Can read, speak, write or otherwise understand multiple languages, potentially including Aklo, Atlantean, Benthic, Cimmerian, Enochian, Lemurian, Muvian, Pnakotic and/or Valusian, as well as ancient/lost human languages, and those of Mythos beings. However, they may be ignorant of modern human languages.
• Hide 50%, Sneak 75%, Mmm. Mmmmm? Mmmmm! Mmmm. 80%

Spells: at the cost of 1 magic point, Nagpa can cast the following spells (these cannot be learned by/taught to non-Nagpa, although a Yithian possession would be able to use them until it left the host):

• create flames sets a non-living, flammable object alight, which will burn as normal until extinguished; it must be visible to the Nagpa. For 3 magic points and an uninterrupted 10 minutes of concentration and visualisation, the Nagpa can cause to ignite an object not present that is has seen within the last 24 hours.
• corruption causes a visible non-living object to rot/rust/disintegrate within the space of a combat round. For 3 magic points, it can do as with create flames. 
• darkness plunges a 'room' into normal darkness; extant light sources are not extinguished. The room must be visible to the Nagpa, but does not need to be completely enclosed (it could also be a tent or a clearing, at the Keeper's discretion). Nagpa can see in the dark.
• paralysis costs 1 magic point per person affected, and the Nagpa must overcome POW vs. POW to do so (it can expend further magic points to boost its effective POW). Victims are unable to move for 1-4 combat rounds, but are aware of what's going in. Nagpa usually use this spell to make their escape or to cast more spells.
• illusion can be static or animated, but makes no noise, is the same temperature as the air, and has no scent; it is dispelled by the touch of naked flesh. The illusion cannot cause actual harm, but how you react to it -thinking it to be real- might.
• (optional) telepathy within-sight universal translator mental communication in case you don't want the language barrier to be a problem/puzzle, or give this power to devices (ill-fitting circlets, for example) found during the scenario.

In addition, a Nagpa will know or have easy access to spells at least numbering the average of their INT and POW. One of these is likely to be Resurrection, for its reverse may hold the key to final dissolution if modified properly. Brew Space Mead and Summon/Bind Byakhee are also common, as most Nagpa will have at least once hurled itself into a star or black hole.

Habitat: anywhere there are the scientific and sorcerous resources for their researches, preferably far from humans: lost cities, abandoned Mi-Go outposts, remnants of Atlantis etc.

Sanity Loss: 0/1d6 SAN to encounter a Nagpa.

Commentary.

I like fiddling with the Mythos canon because it's become so concentrated and static (and this is a criticism that has been levelled all the way back to Derleth), so this is not for purists. 

The Nagpa/Skeksis conversion came about as a side-effect of working on a Random Monster table for Interstellar Space (as the next step on from The Low Moon Itself) - I had a train of thought about non-canonical Byakhee looking like a Skeksis space bee. The Nagpa conversion is the first stop on the way there (and returned the favour with the Space Mead-chugging, Byakhee-wrangling, nuclear-heart-of-the-Sun-plunging idea).