The antichrist is the opposer to everything holy in the world and the one who will control the earth when tribulation comes. He will place himself before God and everything else he will proclaim he is god. He will fool the majority of the earth's population with his deception and charisma, and will attempt to wipe those whom oppose him off the earth...but after his 7 year rule over the earth he will be destroyed along with satan and all evil.
And no I really doubt marilyn manson is the antichrist. the antichrist will be someone of immense power and will probably be good looking, intelligent, and charismatic people will follow him because of his deception and leadership skills and he will rise to power.
marilyn manson is a freak the real antichrist will be the least likely person claiming to work in the name of peace and general good but sowing the seeds of evil.
It is highly possible that there is always an antichrist on the earth due to the fact satan does not know when The Day of The Lord will come.
At the end of tribulation Jesus will throw the antichrist into the lake of fire(hell).