A strange organism in many ways. They are like bodiless minds, but incapable of thought. Normally, they cannot be seen. They can be faintly heard, with a sound like a swarm of flies, and animals can certainly sense them. They are parasitic; they take over the mind and body of other creatures. Hivers normally target powerful creatures, like tigers, and when attacking Humans, aim for powerful ones such as Wizards and monarchs. The people and things a hiver consumes begin to become incredibly powerful, eventually dying insane. The reason why they do this seems to be because they're afraid of the whole universe. They are completely and utterly aware of everything around them, knowing every single blade of grass, seeing all the colours in a tree. They envy humans because, in comparison, we are nearly blind, with the amazing talent known as 'boredom'.
The chief source of information on Hivers is the book by Sensibility Bustle, sometime Professor of Magic, who set out to capture one. His work provides a good resume of what was known about them to that point, and since it trails off into dribbling, paranoid, incoherence, a good example of why they should be avoided. Research students at the Unseen University are advised to read it.
According to Prof. Poledread, Hivers were formed in the first seconds of Creation. They are not alive, but have the shape of life. Most often they end up at the bottom of deep seas, or in the belly of volcanoes, or drifting through the hearts of stars. They have no body, no brain, no thoughts, but they do have the ability "to crave and to fear." They also have memory.
According to a Hiver, it is naked awareness. Without the limitations and protection of a "me", a self, it is under a relentless noonday sun of sensation.
The Hiver is in some ways similar to the Auditors. Unlike Auditors, they have no known cosmic function, though Bustle speculates that they may have been a driving force in evolution.
In fact, in the context of living things, a hiver becomes like a kind of hermit crab, or in the example given by Bustle, the Hermit Elephant of Howondaland. It seeks safety, in the strongest possible refuge. It is not a parasite, does not intend to consume its hosts, but in seeking to reinforce them by giving them the power to fulfil their wishes, it destroys them.
Tiffany may have been the first person on record to have communicated with one. In helping the Hiver, she gave it a story by which it could understand living things, and how it overlooked the most important part of human beings. She helped it find a point in the midst of its multitude of voices that it could call 'me', that could make the journey across the desert.
The Hiver and Tiffany
Tiffany is taken over by the Hiver twice. The first time she is caught unawares, at a moment when she says "See me", something she should not have done without learning how to protect herself. As Granny Weatherwax said, "She's learned how to Borrow, has she? Or she's been Borrowed!" Although the Hiver sees the world through Tiffany, and she sees the world under its influence, there is still a part of her that it cannot reach. This part can write "Help me" in chalk on the dairy table without the rest of her being conscious of it. Something similar happened in the drome-dreams of The Wee Free Men, when odd images came to her in the dream from herself, trying to wake herself up.
She evicts the Hiver twice. The first eviction is by surprise, when she again says "See me", and finds that the Hiver has been put outside her. It came straight back in, though, and a struggle ensued. Miss Level came into the room at that moment to reprimand Tiffany about something else, and the Hiver killed her instantly. With Tiffany stretched out on the floor like a dead person, the Feegles, who despite their intense earthiness, are magical creatures, find a way into the landscape of her mind, and it is her last refuge, the image of the old shepherding hut on the Downs. It is perilously close to the end, but the un-Hivered part of her communicates with the Feegles, the words appearing as chalk writing on the side of the old hut: Sheep's wool, Turpentine, Jolly Sailor. These three things are a tangible memory of Granny Aching, and in their inimitable style the Feegles fetch them. Strengthened by the scent-memory of these, the second eviction of the Hiver is through a rising up in Tiffany of that self that is the same as the Chalk Hills. This self has power, and grasps the Hiver and throws it out.
Granny Weatherwax brings Tiffany back to herself, putting her back to the work of her daily chores, sifting among the voices which speak in her to find the one which is really Tiffany, needling her to bring her un-self-pitying Third Thoughts to the top.
Tiffany is shocked to see what she has done: the cruelty to Petulia, the tormenting of Brian, the theft from Mr Weavall, the "killing" of Miss Level. All the Hiver did was give power to thoughts or wishes that were in her, and she fears that these are the real her. Granny shows her how these possibilities are in everyone, and that they were unleashed because the important bit of her, the bit of her which the Hiver could not reach, was shut away. "Learnin' how not to do things is as hard as learning how to do them. Harder maybe."
Tiffany knows that there is a bit of the Hiver in her, and a bit of her in the Hiver. Each knows something about the other. She knows that they have not been thinking about it the right way, but does not yet know what that would be.
Something worries away at her Third Thoughts. The Hiver gives power to your wishes. In fairy stories, people are always being given three wishes. "What is the third wish?" She asks one person after another. At the last moment, she finds Granny, who tells her that the third wish is to undo the damage that has been caused by the other two. Make this not have happened.
Through her connection with the Hiver, Tiffany realises that although it seems to be bent of attacking her, it is not inherently evil. When it comes the third time, she catches it in a real shamble, the first she has ever made, but also says to it, "Welcome. You are safe here." Through its connection with her, it has also learned from her. It has not come to attack her, but has come with a wish of its own.
In relationship with Tiffany it realises what it wants is to learn how to end, to be shown how to die. In this it is not unlike Lady LeJean, the Auditor who tasted life.
Accompanied by the irrepressable Rob Anybody, Tiffany finds in herself a way to show the Hiver through the dark door, and to set it on the way across the desert. She tells it a story in which it can believe. She says that this is what she is doing, and does not tell any lies. In helping the Hiver to find its 'me' she gives it a name, not unlike that of Miss Level's house-ghost Oswald, in this case Arthur. She crosses the threshold of the dark door herself to help the Hiver cross. When Rob Anybody says to her that he would not trust the scunner, she says there is part of her in it. She would trust that.
At the end of her encounter in Wee Free Men with the Queen of Fairyland, having seen the Queen transform through many monstrous shapes, but still not leting go of her, Tiffany saw her as she was, small and grey, like a monkey. Telling her to go, and not to touch her land again, she said to the Queen, "But I hope there's someone who will cry for you. I hope the King comes back." There is an echo of this in the Wintersmith, where she would herself cry later for the wintersmith who wanted to be human. You can say "No fight, no blame", but it is more than that.
In Alan Garner's fantasy novel, The Moon of Gomrath (1962), there is a sentient being, taking the native form of a nebuluous black cloud with two glowing crimson eyes, which will possess and inhabit the body of any living creature until that creature dies under the intensity of the spirit posessing it. This creature then looks for another host, and carries on serially possessing living creatures. This may be a version of an old Celtic spirit of evil, rather than something invented by Garner.
Garner's "Brollochan" is to all other intents and purposes identical with the Hiver, in purpose, practice and result. (Although the souls of the Brollochan's victims are forced into Anbarn, the Celtic hell: with the Hiver, some shreds of sentience and independence live on). The only things that defeat the Brollochan are the witches - an altogether darker and more malevolent creation in Garner's world - and the Elves, who have more in common with Tolkien's vision than Pratchett's (although Elves in Garner's world are dwarf-sized). In a Celtic exorcism, the Brollachan is forced from the body of Susan (heroine and in all but name, an apprentice white witch) and the noise is like that of thousands of flies...
Perhaps TP read Garner in his childhood and this is an unconscious borrowing: or like Garner, he has gone back to the same root sources in Celtic myth, which explain the similarities between the two creations? As TP himself put it, when the ignorant accused him of plagiarising JK Rowling: "Look, we're all fishing from the same stream here here!"