Luke 8:30-31

Jesus was probably asking the name of the demon who indwelt the man for His disciples’ benefit. "Legion" was not a proper name but the name of a Roman military unit that consisted of about 6,000 soldiers. The name "Legion" communicated that thousands of demons indwelt the man (cf. Luke 8:2; Mark 5:13). The "abyss" refers to the final confinement place of the devil and his angels (cf. Romans 10:7; Revelation 9:1-3; Revelation 11:7; Revelation 17:8; Revelation 20:1-3). The Jews thought of it as a watery deep below the earth (cf. 2 Peter 2:4; Judges 1:6). Only God can send demons to the abyss. This is another indication that the demons recognized Jesus as God. The disciples should have learned from them.