Adherents of the Unspeakable Oath

Adherents of the Unspeakable Oath (previously called Unspeakable Possessors) are creatures created by Hastur when it collects on a binding oath known as the Unspeakable Promise. Adherents have grey-green, scaly skin, with a bloated parody of a body. Their limbs also become boneless and fluid, like an octopus'. Like Hastur itself, they are only active when Aldebaran is in the sky at night, collapsing into a coma either when it sets, or the sun rises.

In the RPG...

There is a 2% chance per year—non-cumulative—that Hastur will collect on his oath and turn the caster into an Adherent. If the caster dies before collection, Hastur will possess their nearest blood relative within 1D6 days. Once transformed, their DEX and STR is doubled, CON is tripled, and SIZ is ×1½, while INT and POW simply become 75 and 175 respectively (Note: DEX used to remain unchanged). 

    Previously, one of the Adherent's attacks caused instant death, which was quite severe considering its 85% Fighting skill. Thankfully, this ability has been removed, leaving it with its other attack, whereby it uses its mouthed fingertips to suck out the victim's bodily fluids. This does an average of 5 damage per round, and the drained points are multiplied by 5 and divided between the creature's STR and SIZ.  

    Unlike the similar Spawn of Nyogtha, Adherents do have one fatal weakness, and that is the aforementioned connection to Aldebaran. When they fall comatose, they lose an average of 50 from STR and SIZ. If either stat reaches zero, they will die and dissolve.

In a video game...

They only have 6-point armour and no immunities, so are not unkillable, so could probably work now that they no longer have their Insta-Death ability.

In a film...

Though I'm not familiar with the source material—The Return of Hastur—the Adherents of the Unspeakable Oath could be pretty horrific under the right direction.