The answer to who captures is now obvious. The beast and the false prophet were captured by other evil spirits. These spirits threw them into the lake of fiery lake of burning sulphur.
In the beginning, the spiritual powers of evil rebelled against God. That set in train a series of events that released destruction and evil on God’s good earth. Since then they have always had rebellion in their hearts.
They gather for the great spiritual battle and attack the beast and the false prophet, because they believed that these spiritual powers had made a serious mistake. They rebel against them and capture them.
We often assume that spiritual powers of evil are united in a common purpose. That is not true. Wars and jockeying for power are common in the kingdom of darkness. The strongest spirits hold power, but other are always looking for opportunities to usurp their powers. The big spiritual battle was a rebellion against the beast and the false prophet.
Many evil spirits rebel against the powerful spirits that had controlled them. These rebellious spirits throw these big-name spirits into the fiery lake of burning sulphur and organise for them to be tormented.