[ Предположительная тональность: C ] I'm pretty sure the entire song follows this pattern: Dsus2 - Am - C - G I think the actual chords might be a little bit different, but these sound alright if you play along as you listen. There's a bit more to it, such as some bass notes here and there but hopefully this will inspire some people to make a better tab.
https://primoaccordo.net/delerium/a-poem-for-byzantium.htm