The Grim is an octopus that resides inside the house of Port witches. It is used by Keeper Apprentices to prove their skills. They cut a piece of Grims tentacle and try to return it back to Keeper Gottage.
In Syren, Wolfboy went to house of Port witches and told them he wanted to feed Grim and asked what would they give to him in order to feed it. Witches tried to give Lucy Gringe to him, but Wolfboy refused to feed Grim by her.
After witches went to upstairs, Wolfboy told Lucy to scream, and meanwhile he feed Grim by cat and cutted piece of its tentacle, but forgot to hide it and so witches smelled Grims blood and thought that Wolofboy killed it.