Baghdad, Iraq - More details have emerged about the shadowy cult whose followers fought Iraqi and US forces in a day-long battle in southern Iraq on Sunday.
Iraqi officials say 200 members of the group - which calls itself the Soldiers of Heaven - were killed in fierce fighting near the Shia holy city of Najaf.
A well-armed group, a charismatic leader and an audacious plot to attack a holy city and kill its religious leaders.
If a novelist had invented the story of the Soldiers of Heaven, it might have been dismissed as a dark fantasy.
But an account of the bizarre drama in southern Iraq, albeit with puzzles and inconsistencies, has now emerged from Iraqi officials and eyewitness accounts.
Messianic belief
A young Shia leader, Dia Abdul-Zahra, had gathered hundreds of his followers, including women and children, in an encampment a few miles north of Najaf.
They were well armed and had come to believe that Abdul-Zahra - also known as Ahmed Hassan al-Yamani and Samer Abu Kamar - was the Mahdi.
According to Shia belief, the Mahdi is a Muslim messiah who disappeared hundreds of years ago and whose return will usher in an era of peace and justice before the end of time.
Abdul-Zahra and his followers regarded the religious leadership in Najaf as illegitimate.
Iraqi officials say their extraordinary plan was to enter the city in the garb of pilgrims, declare that the Mahdi had returned, and assassinate Ayatollah Sistani and other senior clerics.
All this was to happen on Ashura, the holiest day in the Shia calendar.
Instead, the Iraqi authorities seem to have had a tip-off. According to their account, they attacked the encampment and foiled the plot.
At least 200 of the Soldiers of Heaven were killed. Officials insist these included the group's leader, and news agency pictures show a dead man closely resembling him.
Among those captured were Sunnis as well as Shia and foreign fighters as well as Iraqis.
Unholy alliance
Iraqi officials have claimed the group had links with the militant jihadists of al-Qaeda.
Given that Sunni jihadists are fiercely anti-Shia, this seems unlikely.
They also say the group was working with former Baathists.
It seems the former Iraqi ruler Saddam Hussein did try to use a Mahdist faction as a weapon against the traditional religious leadership in Najaf, whom he saw as a threat.
Whether those links survived the fall of Saddam is not clear.
Shia divisions
Iraq's Shia-led government may have an interest in promoting the idea of such an unholy alliance.
It may want to deflect attention from the embarrassing fact that the majority Shia community is riven with factions and divisions.
The authorities may also have exaggerated their own military success.
The signs are that they underestimated the strength of the Soldiers of Heaven and had to call for urgent American air support.
History of the Mahdi
There are both Sunni and Shia versions of the Mahdi tradition.
Throughout Islamic history, Muslim leaders have risen up in rebellion claiming to be the Mahdi or to be acting in his name.
Britain's General Gordon was killed in Sudan in 1885 during a Mahdist insurrection.
In Saudi Arabia in 1979, Sunni militants took over the Great Mosque in Mecca, claiming the Mahdi had returned.
But Shia attachment to the Mahdi tradition is particularly potent.
One of the most powerful Iraqi militias (which has no known link to the Soldiers of Heaven) is the Mahdi Army of the radical cleric Moqtada Sadr.
For many Shia, the idea of a Muslim saviour who will end suffering and oppression has a special appeal.
At moments of crisis and chaos, they are more susceptible to the idea that the end of time is at hand.
Iraq is experiencing just such a crisis.
And in current circumstances southern Iraq - the Shia heartland and traditionally the poorest and most neglected part of the country - seems fertile soil for zealotry.