Anime where a lot of people had to defeat a demon boss but the main character defeated the demons then the demon boss gave his power to him by going inside him. There was a triangle pyramid with a door on it that opens in the center but when he tried to go inside people tried to kill him with a big fire ball but he deflected it.
Then when inside the doors there was a tree in there.
In the background there was smoke when the boss came out, it stayed there until he defeated it.
There were long rectangular shapes coming out of the floor at the place where they were fighting.
I don't know what the main character looks like for some reason.
The boss fight starts 10:00 minutes in to the video.