^9:1 And the fifth angel sounded, and I saw a star from heaven which had fallen to the earth; and the key of the bottomless pit was given to him.

And the fifth ANGEL sounded,

This is the first WOE of the WOE, WOE, WOE in Revelation 8:13 (the previous verse). It will take eleven verses to describe this TRUMPET blast. Finally, in verse 12, this first WOE is past.

and I saw a STAR from heaven which had fallen to the earth;

Since this STAR "had fallen" indicates the fall took place previously. Perhaps, this star is the one in the third TRUMPET, Revelation 8:10. The most common interpretation is the STAR is a fallen angel or the devil himself.

and the key of the bottomless pit was given to him.

Here is where things get interesting. Christ has the keys: Keys of Death and Hades, Revelation 1:18, Key of David, Revelation 3:7. So it is as if Christ is chiding the devil by saying, "Show me what you have in the bottomless pit that you could possibly use to overthrow Me." Just the act of the devil having to take the key shows the superiority of Christ.

Now let's consider "the bottomless pit (abyss)". It is self-contained because it is lockable. Otherwise, there would be no need for a key. However, the word "bottomless" contradicts the idea of self-contained. So the only conclusion is that physical boundaries are subject to physical laws, and spiritual/evil boundaries are subject to different yet lockable laws.

There are two types of demons. Those who are free to roam on the earth. These are the ones Jesus cast out throughout His miracles period. Those who are bound in the pits of darkness for sins committed as Peter says in 2 Peter 2:4-9. They are being preserved for judgment as Jude 6 confirms.

Some think that the Nephilim of Genesis 6:1-8 are some occupants of the bottomless pit after they were drown in the great flood. It is a place so horrible that even the "free-roaming" demons do not want to go there, Luke 8:26-34.