Clear Plane B
Keep the Magical Lantern stashed away and head through some mist to the north to seemingly travel back to where you last saw the ghostly images, near where you killed the last pair of Primal Giant Spiders. This is, in fact, a different version of the Other World than you’ve been traveling through, and here it’ll become apparent how this place works.
There are actually two nearly identical planes that you’ve been traveling between, let’s call them Plane A and Plane B. In Plane A (the one you originally entered) there are two small areas in the center, with the northern one housing the Old Gnome and the southern one (where you initially appeared) being home to Primal Giant Spiders. In Plane B (where you saw two scenes with the Guardian of the Bloom) these two areas are connected. Both have a side area to the east that connects the northern and southern halves, while to the west both have a ruin, the pool of bitter regrets, and the trapshroom-lined hill.
You are now in Plane B, meaning there should be new critters to fight. Sure enough, head downhill to the southwest and you’ll find two groups of Primal Giant Spiders. On the plus side, the trapshrooms aren’t traps anymore, so no line of easy experience for you.
Kill the arachnids, head downhill, turn east and reach the clearing where in Plane A two Owlbears lurked. No such threat is present here now, but if you head north to the pool of bitter regrets you’ll find a pair of Primal Manticores. Lure them south and bottleneck them near the ruins, as they’re much easier to deal with one at a time.
Equip the Magical Lantern again and head north past the pond of bitter regrets, then turn east past some more mist to return to where you saw the will-o-wisps accosting the Guardian of the Bloom. Continue east through some more mist to find some Primal Giant Spiders. More importantly, you can find a secret passage [Perception 25] to the east. This will take you to the ledge where the Treant-Enlarged Owlbear resides… and the treasure it guards.