“Explain to me what it is,’ Gaunt said to Landerson bluntly. Landerson shrugged. ‘I can’t… I mean, I’m no magister, no sorcerer. I don’t understand the workings of Chaos.’ ‘Try!’ Gaunt snapped. ‘It’s an expression of the warp,’ Plower said. ‘That’s what I was told. The Archenemy has branded our world in every way, even the atmosphere. A glyf is the way Chaos makes its mark on the very air. A glyf is a thought, a concept, an idea… an utterance of the Ruinous Powers somehow conjured into solid form. Some say they’re sentient. I don’t believe that. Glyfs are Chaos runes, sigils, symbols, whatever you want to call them. The ordinals summon them into being and release them to watch over the populace. They drift, they patrol, they lurk…’ ‘Great,’ cut in Curth sourly. ‘But what do they do?’ Plower looked at her. ‘I suppose you could describe them as tripwires. Sensors. Alarms. They react to human activity. I’ve no idea how. Certainly, they respond to imagos. If they detect anything unconsented, they… they react. They summon.”
―
Dan Abnett,
The Lost