Muggles' Guide to Harry Potter/Magic/Murtlap

Overview
Essence of Murtlap Tentacles is shown to be a powerfully healing and soothing solution.

Extended Description
When Harry serves a detention with Professor Umbridge, he is required to write lines with a quill which creates a wound on the back of his hand. After a week and a half of this, he is in considerable pain. Hermione gives him a bowl of Essence of Murtlap Tentacles, which he finds extremely soothing.

When Lee Jordan falls afoul of Professor Umbridge and is similarly abused, Harry recommends Murtlap to him; Lee, then, evidently suggests the same cure to Fred and George when they are suffering self-inflicted boils in a rather private place.

It is in Fantastic Beasts and Where to Find Them by "Newt Scamander" that we learn that the Murtlap is a rat-like creature with a growth on its back that resembles a sea anemone.