Sirius O. B.

yIt's the morning after a full moon. Remus turned unexpectedly, as he didn't realize that it would be that night because they were having fun and lost track of time, and Sirius had to rush him to the Whomping Willow under James' invisibility cloak.*

They made it just before Remus turned. Sirius couldn't leave the shrieking shack, it was far too risky. He had to stay with Remus.

He hid under the invisibility cloak the whole night, but not before Remus cut his shoulder. When Remus woke up, he found Sirius sleeping against the wall.