The way this tree trunk grew

Could it be one of those parasite plants that grew around a tree, killing it, and said tree rotted away and left this 'exoskeleton'?

It's obviously a magical door to another realm

That's not one tree.

That is a fig tree! (there are about 800 species of fig) Figs are parasitic plants that grow on/over another tree, eventually killing said host. By the time the host rots away, this gorgeous, woven "trunk" is sturdy enough to stand on its own! Really fun to climb

That's a strangler fig