Elishar Feverfew was a once mortal pirate, who somehow managed to gain entry to the Border Sea across the Line Of Storms.
One in the Border Sea, Feverfew appears to have trained himself in House Sorcery as well as how to use magic. Exposure to these changed him from a mortal into something neither Mortal, Denizen or Nithling.
When Wednesday planned to denounce her role as Trustee, Feverfew was employed by the Morrow Days to steal the Third Part of The Will and guard it from the Rightful Heir as well as Wednesday herself. In order to do so, Feverfew was created a worldlet inside an immaterial bubble which was placed inside Wednesday, who by this point had undergone her transformaion into Drowned / Whale Wednesday. Feverfew continued to pilage the Border Sea and also recruited other pirates from ships he captured. He was largely unhindered until Arthur arrived in the Border Sea with the task of freeing Part Three of the Will.
Feverfew was descriped as a practitioner of House Sorcery of such a nature that even Sorcerors officially trained in the Upper House had little knowledge of. Examples of this include the red substance on the bouy, an acout of him causing a ship's rigging to choke the life out of it's crew and causing the wood of The Moth to bite at the crew. His mastery of Nothing allowed him to poison the grapeshot fired from the Canons of his ship, Shiver. Master Yongtin, a medical raised rat commented that it was strange how he had manged to make it stick together.
When Arthur entered the Border Sea, he accidentaly summonded Fevefew to one of his treasure markers. Arthur was then picked up by The Moth which narrowly managed to escape the pursuing Feverfew through the use of an Augurie puzzle. However when the ship returned to the Border Sea, Feverfew was waiting on the other side, ready to capture the ship. Around this time, he sucessfully captured The Flying Mantis, a notable accomplishment as it was one of the Architect's original 49 ships.
Feverfew took them back to his secret worldlet and when Arthur arrived via Drowned Wednesday's stomach, engaged in a wager as to who could kill the other. Arthur sliced off his head. Feverfew's skills allowed him to survive this (like most denizens) however his disembodied head would simply fly back to his body and reattach itself, despite interference from soil and a blade resting on the neck stump.
Arthur sliced his head off again, but before it could reattach, Suzy smashed it to the ground with a branch, and when it once more tried to reattach, Leaf kicked it into the Nothing mud of the Hot Lake. His head managed to survive long enough for the skull to shout "Let Nothing remain," causing Feverfew's worldlet to begin collapsing.