Yes he is.
Revelation 6:1. Then I heard one of the four living creatures say in a voice like thunder, “Come!” 2 I looked, and there before me was a white horse! Its rider held a bow, and he was given a crown, and he rode out as a conqueror bent on conquest.
That 12th imam the Mahdi is the biblical Antichrist because when he comes according to Islam theology he will shutter crosses symbol of Christianity meaning his entry will Mark the annihilation of Christianity and establishing Islam as the only faith on Earth. This Mahdi the 12th imam as they call him is the biblical Antichrist
Bloody nyoko