against the immortal dragon Seath the Scaleless

Yup. another Dark souls Prepare to Die picture. this time it's a picture of a random sorcerer who is fighting against one of the coolest bosses called Seath The Scaleless. tricky boss against you to fight at, try and find his weakness to get him to be able to defeat. he's immortal, only because of a certain thing. what is that thing in this picture?