首楞嚴義疏注經 8.474

8.474

即前誑習為因也。為獲利譽，多懷異謀，矯現有德，罔[曰/月]於他，令他暗昧不曉己事。洎受鬼形，憑幽託暗，魘惑寐者，故名魘鬼。

This is caused by the previously mentioned habit of dishonesty. To gain profit and fame, they often harbor devious schemes, falsely presenting themselves as virtuous to hoodwink others, causing others to be benighted and unaware of their affairs. When they receive ghost forms, they rely on gloom and lodge in darkness, oppressing and bewitching sleepers with nightmares, and are therefore called nightmare ghosts.