[DF] Deep Ones

[b-dog]

This race was thought to have started in the material world as tiny seeds of extradimensional material left by the the Great Cthulhu. These seeds settled into the ocean and combined with material reality to become the underwater race called Deep Ones. They exist partially outside of material reality and so thus can build extradimensional palaces deep below the ocean waves. They can reproduce young but it is difficult due to their extradimensional nature and so often prefer to have humans bear them children which appear as normal humans or other races until they transform into a deep one and return to the sea. It is thought that Cthulhu placed them into material reality so that they can garner energies from prayer and sacrifice so as to allow Cthulhu entry into the material plane. Cthulhu is composed of far more extradimensional material than deep ones so he requires far more energy to all him to exist in the material world and even then he must sleep until the stars are right.

[Jerander]

Have you seen Kenneth Hite's (author of GURPS Horror) book, Hideous Creatures: Deep Ones? You can find it here. It's not for GURPS, but it might be inspirational for your treatment.

[b-dog]

An encounter with Deep Ones in a dungeon is usually very dangerous. They dig tunnels with doors of extradimensional archetecture and usually disguised as being natural stone formation. These doors do not radiate magic and are extremely difficult to detect. The tunnels often lead to underground seas or lakes and often have aquatic creatures guard them. They are innately psionic specializing in telepathy which they use to communicate amoung themselves and thus alerting the rest of their brethren in one spots an intruder. Their leaders will be much larger than the others as they never stop growing as they age. Some may have much more powerful psionic powers like telekinesis or mind blasting. The viewing of Deep Ones is dangerous to mortals as well due to their extradimensional nature. They are composed of an amount of extradimensional energy that twists their fish men bodies into nightmarish horror. Some are priests to the Great Cthulhu and they have been granted special powers and the ability to summon other extradimensional creatures.