The core left behind by a Breacher Primus.
There is no way to exterminate this life form. One reduced to nothing but its core will resurrect the next day or in decades. Luckily, it is neither aggressive nor intelligent, so Fontainians can just keep their distance.
According to the Marechaussee Hunter who accompanied the White Cavalier of Protection, countless Breacher Primus emerged after the horrible battle in Elynas, but none assaulted the two.