(Another idea I had for Zak S. open-source RPG project, Gigacrawler!)
The Rhythm of the Layers- Sprawling across gravity wells, dimensional rifts, and who knows what else, as it is pushed and pulled by cosmic tidal forces the entire Gigastructure resonates on a subsonic frequency that is commonly known as the 'Rhythm of the Layers.' Although some entities are immune to this phenomenon, the Rhythm can have a baleful effect on sentient creatures, especially with abnormally-long lifespans (such as elves and genetically altered human immortals)- for every hundred years a Crawler has lived, he/she should make a Sanity check with a cumulative -1 to the die roll for each successive century. Failure means that the Crawler will spent the rest of its existence dancing throughout the Gigastructure in a trance-like state, roaming with other affected Crawlers in great capering hordes like the Bacchae of ancient Greece, deranged and deadly.
It is said that some Crawlers worship the Rhythm as a kind of deity and use technology to amplify and/or internalize it with cybernetic implants. If the mental image of some kind of Disco Borg entity doesn't chill you to the marrow, I don't know what will!