The last days are those previous to the Doomsday and mainly described in the Book of Revelation. There will be some signals during that time, as described in Matthew 24:3-51, Mark 13:3-37 and Luke 21:7-36. Some of the signals are also described in the Book of Revelation. These are studied by Christian theology.
Concerning Christian demonology, the Devil will be given a great power in that time, and false prophets and Messiahs will come. As the Messiah was named Christ, an anti-Messiah is an antichrist, but the anti-Messiah would be the Antichrist (a biblical term used solely by John in 1 John 2:22 and 4:3). Many people will believe in the false prophets and they will persecute Jesus' followers.
But the Devil will be allowed to torment men (good and evil) with demons (200 millions, as told by John in Rev. 9:16) causing wars, plagues, famines, and every type of catastrophes. These demons are described as monstrous beings coming out from the "abyss", and led by four angels (supposedly fallen ones).
Later will come the Beast (the Devil), described as a dragon, and kill two envoys of god that will be allowed to torment people causing the lack of rain, the changing of water into blood, and more pestilences. People will give thanks to the Beast.
Then will come a second beast (either a demon or a miraculous man), with a blasphemous name (not explained), to which the Beast (capitalised) will give power and allow it to make wonders, and people will worship both beasts, the dragon and his minister (described as having seven heads with ten horns, the body of a leopard, the feet of a bear, and the mouth of a lion), and the beast will speak blasphemies (not explained, but supposedly against God, Jesus and the Holy Ghost).
Finally a third beast will come, with two horns like those of a lamb but the speech of a dragon. This beast will be given authority by the second beast, make more prodigies, and make men worship the second beast. This third beast will command men to make a statue of the second one, to which by power of the Devil will be given life, and the "idol" will make kill all those that do not worship it. The third beast will order that every person is marked on the forehead or the hand with its (or his) symbol, name, or number of the name; and here we are speaking of a man, as told by John in Rev. 13:18 (the number of the beast is the number of a man and it is 666). See also Number of the Beast (numerology). This third beast is also referred to as "the false prophet" (Rev. 13:14).
According to John (Rev. 16:13-14) the three beasts had inside three "spirits of demons" with the shape of a frog.
Then God will send his angels to cause more plagues and exterminate those who worshipped the beast(s) and they will be thrown into the fire. The beasts will be sent to Hell too. Jesus will come for the second and last time, battle with the Devil (the Beast), defeat him, and also cast him into the abyss. All those that were condemned, demons and humans, will remain in Hell for all the eternity and none of them will be forgiven.
Note that John uses abyss, fire and Hell as synonyms, and the numbers he gives (as all numbers given in the Bible) are symbolical.
The seven horned heads of the second beast described by John seems to have a reminiscence with Leviathan by its appearance, so it could be referred to as a demon, but it was not made clear, being all the book full of symbolisms that many exegeses intended to decipher, but nothing too much understandable was achieved.
To the third beast John said it is a man, but the number is used either to investigate by means of numerology who could be the Antichrist (from Nero to Adolf Hitler and Saddam Hussein, including Martin Luther who was a Lutheran theologian, and a mythical being like Melusine).
See also