it's a port of an old region mod
People who modded 1.5 would understand.
Or you coulda just read the freaking description

Apologies for missing this, apparently I don't get notified for new threads~

I've updated the description to make it a little more clear what this is. In addition, I figured I'd elaborate a little more here.

New Horizons is the name of a very large region expansion mod that was mostly made by somebody who goes by 'FishSwallower.' Over the course of development, many of her earliest regions fell behind the quality standards of the later ones, and so she decided to scrap them from the pack.

This mod is all of those scrapped regions, although they've been heavily polished by me and Solar. Including redoing all the creature spawns completely, changing the most egregious platforming, making several new rooms, and much much more. It's still rough in many places, but many of the edges have been smoothed out.