This type of Ghost has been briefly mentioned towards the end of The Book of Nighthunting, which has just been completed* over here or you can watch the clipshow of it here:
[*Edit to add: There may be further entries slotted into it at a later date, but the main structure of it is obvious now – There are three levels of Ghost, insubstantial, half-formed, and those able to manifest a somewhat solid body and manipulate their environments. There are many different types of Ghost on each level, (a bit like Pokemon, only horrifying and hazardous to Life). After Level Three, Ghosts transform into Witches, which are the worst kind of Dead creature a Nighthunter has to banish.]
Saying all of that, The Llewmayn is something special, as there are only ever two or three in existence and this type of Level Three terror has a substantial part to play in “A Dark Heritage: The Nighthunter” (available on Amazon!)
So… what is this creature?
The Llewmayn is a spectre and also a gigantic form of prehistoric jellyfish. In the world of the Dal-Rhiatan Empire, it has been alive since the dawn of time, and it possesses a frightening amount of intelligence which is so cold it isn’t like human thought. It is telekinetic and telepathic, able to read thoughts, dreams, and memories, and cause its victims to vividly hallucinate memories and experiences from their pasts, often warping those memories or mixing them with other memories or dreams to create an unbearably nightmarish situation.

But what does it do that for?
It does this trick to feed.
Ghosts feed by absorbing neurological or neurotransmitter energy from humans (and to a lesser extent, animals). Highly charged emotions create more of this energy, especially strong negative ones such as anger, terror, and anxiety. Therefore, Ghosts do whatever they can to make their victims Feel Things. Most likely Things they don’t want to feel. The resulting terror or rage gives Ghosts (and the Llewmayn, especially) sustenance.
In “A Dark Heritage…” The Llewmayn is attracted to Cailte’s barely controlled Foundational magic powers, and it gives chase to the ship Gastwrnis whilst Logan and Noran are trying to take her and her father across the White Ocean to Thosea where she will not be persecuted for having those powers. But her magic reminds The Llewmayn of the powers its children once had, in a distant time when it wasn’t almost extinct. It wants to take Cailte as its own, but that would drown her to death.
So, Logan Bartholom (The Nighthunter) and his reluctant companion Noran Cadwallader attempt to reason with the jellyfish Ghost. Logan is convinced he can talk it into leaving because of his amnesia, but after it forces the two of them to relive a disturbing memory of Logan’s and attempts to alter Logan’s scanty recollections of his mother, The Nighthunter finally realises he must destroy the Llewmayn with his sword, Athannor. Unfortunately, by this stage the jellyfish Ghost has already claimed a human life in a terrible way.
And the Llewmayn isn’t the only entity interested in Cailte’s abilities. There’s the forysracos bird, for one thing… but more about that next week…