The idea is pretty simple. Nyrissa created an Anti-Grigori, a young swineherd named Durnik whom she 'chose' for the mystical sword Trollslayer. She approached him while he was herding his pigs near the Tuskwater and told him he was the chosen one, a hero who should rid the land of the troll plague by leading a large army of peasants against them.
She gave him a cursed weapon that detects as +1 flaming giant bane but makes the wielder unable to act while in the presence of giants.
When the heroes return from the wilderness, Durnik is busy recruiting farmers to his cause, weakening the infrastructure of the kingdom. He is friendly toward the characters (unless they are known to be evil), but quite zealous in his endeavour. He's a first level commoner and unless the character are able to stop him, he'll lead hundreds of farmers to their doom,causing a great shortage of food.