Dung Eater will invade you mid-way through his questline or at the end of Blackguard Big Boggart 's questline. As you get a Seedbed Curse, head to the Roundtable Hold and show it to Dung Eater.

Each piece of the set increases the damage of the Omen Bairn and the Regal Omen Bairn by 5%. The damage increase is stackable, so the entire set increases the damage of said tools by 20%.