Anime where people defeat a demon boss and the main character receives the demon's power

Question

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.

Answer 1

It was Gin no Guardian (2nd Season)

At Shinryou Private Academy—an expensive school for wealthy students—one would never expect to find the poverty-stricken Suigin Riku. When he is not working on one of his many part-time jobs to pay his tuition, he can often be found playing the RPG game Dungeon Century, where he has cultivated a relationship with an online friend. However, when Dungeon Century shuts down, he finds out that his crush, the kind-hearted Rei Riku, and his online friend are the same person.

But in the aftermath of this revelation, Rei gets kidnapped and taken into Grave Buster, which is a new online game from the creators of Dungeon Century, forcing Suigin to enter the harsh new world of a pay-to-win game in order to save her. Gin no Guardian 2nd Season continues Suigin's quest to rescue Rei, while attempting to solve the mysteries of this strange game.