First you need to get double jump. *To get it, go up through the Royal Chapel into the towers and go right. *Eventually you'll come across a Griffon (boss battle); from there you'll be close. *Go right a few more screens and you should come across a tall room with two elevators and a door. *Go through the door to activate another Warp Room, and go back and go up an elevator. *Go left out of the room and you should come to a big open air room. *Go all the way left (you'll hit a big arbitrary wall) and you should see the Double Jump Relic. *To get the bat form, you have to go back to the Clock Room, use double jump to get up into the left section (the one that opens every other minute) and go into the Colloseum. *Eventually in there you'll find a Form of Mist (among other nice things). *Once you get that head to the Library, and go to the section that leads to the Librarian. *If you look you'll see that there's another ledge with a Corpseweed that you couldn't reach before. *It'll be a little tricky, but you can double jump there now. *Explore it thoroughly and you'll get the Form of Bat.