Isma

Isma was one of the Great Knights chosen by The Pale King to protect Hallownest. For reasons unknown, she has been transformed or encased in a large tree in the southeast end of the Royal Waterways.

To reach her, the player must first defeat the Dung Defender and then activate the large lever he is guarding. This will drain Isma's Grove of acid, allowing the player to access it by using Crystal Heart to dash through a tunnel to the south of the Dung Defender's boss room.

After running through the gauntlet, the player is given the charm Isma's Tear, which is hanging from the tree's largest root. This charm allows the player to swim through acid and reach new parts of Hallownest.

Trivia

 * The Great Knight Ogrim idolized Isma for her strength and wished for her to teach him.
 * Ogrim's dung statue of Isma in his secret room is the only clue as to Isma's original appearance.
 * Hitting Isma's corpse with the Dream Nail gives the dialogue "...Ogrim... there's no time..."
 * This could imply that Isma sacrificed herself somehow to prevent Ogrim's death, possibly via the acid that floods the area.
 * It could also simply be a reply to Ogrim's request to teach him (there's no time to do so, for whatever reason).