Did you go into a regular, boring, nondescript building and then take the longest elevator ever? Because I did the and it brought tears to my eyes when I finally got down there (also first real souls game, I only played a bit of Demon’s Souls PS3 before Elden Ring)
Edit: also this tree is far beyond any boss in the main game, and insane compared to the other expansion bosses. I don’t even think I need to kill it to progress.