Grieferfish

Grieferfish applies a buff to the targeted player, called "Griefer" (spell tooltip: "Someone thinks you're mean."). It has a duration of 24 hours (real time), but can be removed by right-clicking the icon on your player unit frame or by entering  in your chat window.

Source
This item is looted from Frostdeep Cavedweller/Lunarfall Cavedweller (requires level 3 Fishing Shack in your garrison), and may also be fished up from open waters in. Very rarely, it may be looted when filleting Enormous-sized fish.