How to Acquire the Bat Form Eternal in SOTN

Make sure you have the Leap Stone and the Soul of Mist before attempting.

Now, head to the Long Library.

Keep on heading in until you are going down a lot of stairs. While you are walking near the top will be a thing above you. Ignore it. But, when you find a second thing like it, Double jump up to it, continue from here.

Now in this room, kill the enemy's while advancing.

Once in the next room, continue on while falling where you can. Once you have advanced long enough, the walls will shut behind here and you will fight a boss. His name is the Lesser Demon. This was not easy, so it will probably take a few tries.

Once he is defeated, head on until you hit a grate. Use Soul of Mist to pass, in here is the Soul of Bat.