v1-6: The star which fell from heaven to earth, and he was given the key to the bottomless pit. This star is identified as Abaddon/Apollyon, even Satan himself, v11. Even he has no power except what he received from God Himself.
Through the star, locusts were released onto the earth, which brought suffering to those who were not sealed. Thus, the 144,000 are immune from this particular suffering.
v7-10: The locusts torment men for five months; their appearance and sound is fearful. This period of time, like others mentioned, indicates real events, rather than purely symbolic descriptions.
The locusts, at first site, are not so awful; there are crowns of gold, and human faces. But they bring torment. Like all temptations, there is an offer so different from the reality.
v11: Their king is Abaddon/Apollyon.
v12: The three "woes" begin with the locusts, see 8.13.
v13-16: The sixth angel, and the second woe; the four angels from the Euphrates. They bring death to one-third of mankind. Such terrible condemnation is reserved until the time God ordains.
v17-19: The massive army, some 200 million strong, is released upon mankind. Like the locusts, they have a terrifying appearance. It is the three plagues of fire, smoke and sulphur which exact the death upon millions.
v20-21: Although the rest of the people remaining on the earth doubtless witness the army of horsemen, they stubbornly refuse to repent, clinging on to their sins.