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The Warsingers were a group of rogue Human Psykers that plagued the Imperium of Man in the years of the Great Crusade.

They were supposedly wiped out by the Raven Guard Space Marine Legion though reports came that they had resurfaced on Istvaan III. ¹

As it was a trap laid by the traitor forces of Horus, it's quite possible that the re-emergence of the Warsingers was simply a ploy to trick loyalist forces onto the planet where they were butchered by the rebel forces.

Source

• ¹:Fulgrim by Graham McNeill