Feytor the Tainted is a Chaos Lord of Nurgle created as part of the fluff surrounding the release of Warriors of Chaos during the early "Hordes of Chaos" approach and as part of the build up to the Storm of Chaos campaign. He is one of the Lieutenants of Archaon.
The Champion of Nurgle named Feytor was once a humble farmer in the northwestern regions of the Empire, making him the only one of Archaon's lieutenants to not be from the Northlands himself. During a time of terrible famine and plague, Feytor's little brother caught a sickness, compelling an exodus of their neighbors for fear of the disease. Feytor and his kin remained on their lands to tend to their kin, Feytor praying to any deity who would listen to deliver them. To the family's surprise, though they all began to show physical symptoms of the disease, none of them perished; indeed, Feytor in particular seemed even stronger and hardier than he had before. For this, they gave thanks to whatever kindly god had spared them.
The returning locals were suspicious of them, and eventually drove them away, despite Feytor's protests that they were not plague carriers. Chased away by countless villages and towns, Feytor and his kin eventually settled in a cave in the Grey Mountains. For a time, they lived in peace, then Feytor returned from hunting to find Imperial soldiers had burned his family on stakes. Filled with rage, he attacked them, shrugging off their efforts to wound him and beating them all to death with his bare hands.
From there, Feytor wandered into the north, where a Norscan tribe known as the Wolf Brothers, moved by visions, eventually accepted him into their ranks. He became their chieftain after beating their former leader, Svolos Wolf, to death with his bare hands after growing dissatisfied with his lack of ambition, and the Wolf Brothers grew like the pestilence they resembled, becoming known as the Decayed Ones. It was visions that guided Feytor to seek out Archaon, and he eagerly followed, yearning for revenge against his former homeland.
tl;dr: a farmer got sick, was blessed by Nurgle, and is on a kill all Imperials revenge-fest, whilst often beating people to death with his bare hands.