A majority of my characters die to a level 5 creature, the shimmering cloud, which appears commonly at depths of 250 feet and below.  Owing to its habit of repeatedly casting wreath of fire, it is far more threatening than other creatures of similar level, and a pair of them can easily take down even a level 10+ character if not prepared.

I think it's fine to have a few creatures present an outsize threat — other good examples include the crone and lichling.  After a death or two I learn to play around them.  I just find it curious that these creatures aren't rated higher than they are — even at level 12, a shimmering cloud doesn't feel like an "easy" enemy to deal with.

The difficulty is a little off in places - if the clouds are too hard, there's a lot of stuff that's probably too easy. At some point I need to revisit this. But the clouds are a reason to get high evade, a good ranged weapon, train awareness. And there are routes through the game that will level you up past the point where they're a threat.