The Anti-Christ appears many times in different forms, trying to find a way to conquer the world and bring Armageddon. Apparently, many tried to get it born in mortal realm, but John Constantine stopped it in two occassions.
It was first mentioned during a clash between Nergal's Damnation Army and The Resurrection Crusade, a militant Christian group. Both were trying to fulfill the prophecy that said a child would be born that would change the world; the Army planning the birth of the anti-Christ and the Crusade trying to create a new messiah. John was able to defeat both sides by helping Swamp Thing father a half-human child that cancelled out the birth of the Anti-Christ and dragging Nergal to Heaven and tortured for decades.
Most recently happened when, Fuckpig, entered the body of a gangster's son. The boy whose name is Ronnie Cooper, was killed by an accident, and in order to resurrect him, John was forced to seal the demon inside of Ronnie. But over time the seals wore away and John lost control of the demon. Still in the body of the boy it hatched a plan to impregnate the now-senile Harry with a demon child to bring about a new anti-Christ. The demon was successful in summoning the Anti-Christ, but John managed to threaten the demon to back-off and killed the Anti-Christ with an axe.