Source: Lost Cities of Golarion, pg(s). 12
Midnight milk is an ingested drug created by the intellect devourers of High Ilvarandin – in fact by Lens-Keeper Tiluatchek, with the assistance of the ghast alchemist Bereshkhani. It was created with the aim of enabling the intellect devourers to eat the brains of surface world creatures – their war with the neothelids of Denebrum is going badly, and they sought a means of greatly expanding the number of hosts available to them; midnight milk is the result of their researches.
The drug is created from the waste products of the mu spores which float above the Midnight Mountains in Orv. It is highly addictive, but is (relatively) harmless in itself. Those who drink it experience strange and vivid dreams of exploring the Darklands.
The dreamers are drawn deeper and deeper by an intellect devourer artifact known as the dream lens. The more that is taken, the further the exploration takes them, until eventually they visit Ilvarandin itself. At this point, the dreamer's psyche inhabits the body of one of Ilvarandim's creatures and experiences that creature's life.
At some point, the dreamer will inhabit the body of a creature which is already serving as the host of an intellect devourer. The intellect devourer instantly recognises the dreamer's presence, and heads for the physical location of the dream lens itself. If it can reach there before the dreamer awakens, it can use the power of the artifact to try and eat the dreamer's brain, transporting itself inside the victim's skull if it succeeds.
The poet Vumeshki was one of the first to fall under the influence of midnight milk, and wrote the poem Ilvarandin, and possibly other parts of his poetry collection Songs at Sun's Ebb, whilst under its effect. 
- Tim Hitchcock, Brandon Hodge, Michael Kortes, Jason Nelson, Russ Taylor. (2011). Lost Cities of Golarion, p. 11-12. Paizo Publishing, LLC. ISBN 978-1-60125-272-2
- Tim Hitchcock, Brandon Hodge, Michael Kortes, Jason Nelson, Russ Taylor. (2011). Lost Cities of Golarion, p. 12. Paizo Publishing, LLC. ISBN 978-1-60125-272-2