What does Luke 8:31 mean?

Jesus is ordering a legion of demons—possibly thousands—to abandon the man they are tormenting (Luke 8:26–30). The demons know they must go, but they want to stay on earth, not be sent to "the abyss." We're not sure what "the abyss" refers to. Most literally, the word translated "abyss" refers an extremely deep hole, chasm, or pit, and is often mentioned in reference to death, evil spirits, and enemy powers.

Scripture seems to suggest that when Satan rebelled against God, one-third of the angels followed him (Revelation 12:3–4). Not all of those fallen angels are still on earth. Peter rather vaguely refers to demons who sinned so badly God "cast them into hell and committed them to chains of gloomy darkness to be kept until the judgment" (2 Peter 2:4). These may be the same terrors who will be released during the tribulation (Revelation 9:1–3, 11; 11:7). Some say these are the angels who mated with human women to create the Nephilim (Genesis 6:1–4).

The Bible doesn't give the details, but the Legion of demons fear some version of judgment. Matthew quotes the demons as asking, "Have you come here to torment us before the time?" (Matthew 8:29). In the end, all demons will be sent to the lake of fire (Matthew 25:41). These demons don't want to go yet.