This article contains plot or progression spoilers. Continue at your own discretion!
Daemons are hidden background processes within Mindspikes that help run a framing device, they adhere to 2 rules as explained by BSTRD: they must allow the player to go through the entire memory with the framing device and that the "actors" must comply with the story in the memory that the daemons are in; it is also possible to override the daemons so that they ignore both of the rules, albeit only for a certain period of time.
Daemons can appear in multiple different forms; including hidden background processes or firmaments.
Firmament
Firmament daemons are usually in the form of walls in the thoughtspace the daemon is currently in, firmament daemons keep everything within the thoughtspace inside its confines, and "lies to your spike" about it. Although firmament daemons can also be bypassed via "gaps" as seen in Golem Maintenance.