The Intrusion is the Fear of Insects, swarms and hives being invaded from within. The Intrusion is a massive hivemind,
Centipede Swarm
controlled by a central queen that is the Fear itself. It consists of millions upon millions of beings, from servants, to insects, to cultists, all working together in a horrific colony.

The Intrusion uses human hosts to reproduce and multiply to exponential proportions. Once it reaches a host, it buries itself within every orfice in order for it to lay eggs, where in the process, the victim mirrors the symptoms of paranoid schizophrenia. 

Once the eggs hatch the newborns begin to control and consume the victim very slowly over time.When the insects within mature, they start to infect the brain of the subject, turning their fear and their hopelessness and pain into adoration for what they refer to as the Hive, the One, the Father/Mother, and many, many, more titles. They devote themselves to The Instrusion, and when they give in, either the insects within their brain burrow deep and kill the victim, or they lead the victim into the Fear's realm. 

Its servants are known as Drones who are also the human hosts it infects but develop an insect-like appearance slowly over time the more infected they are. The realm of this being is known as The Hive.