Spoilers: Spirit form is in Mithalas; if you have gotten beast form, take a look at the map and see the white parts in the upper left where you haven't been yet. There's an octagonal chamber you need to get to, and you don't need nature form to defeat the floating priests there. You get to their lair by going up and to the right in the passage with all the houses; you need beast form to get into the area above their chamber. I didn't use nature form for anything in that area except to quickly get through one clogged-up passage full of mutants before the big boss in the cathedral. Most people go to Mithalas, then get nature form; I did it backwards. Part of the fun of a non-linear game.
