Battle starts at 2:26 Thanks for the 800k views! :D
This is the final battle with the demon lord in his full demonic form and is the pre-final boss of the game. The first phase of the battle involves you knocking him off of his platforms and stabbing him through the chest, which is fairly easy. Afterwards, he'll draw his sword and begin fighting like he usually does, although this time you have to stab his chest to land hits because his skin is indestructible; he also has some new attacks such as performing a dark version of your Skyward Strike that you can reflect back at him (I didn't know this when I fought him, so I didn't get to do it). After a few hits, he'll get pissed off and draw a very large sword; break the sword into pieces and stab his chest. You can take your time with breaking his sword since he holds it up for quite a while when he does.