They have very low health and are able to be one-shot by most attacks, but they also have a decent dodge stat and both types can inflict Blight.

Spiders (Webbers and Spitters), even if they're not necessarily demonic in nature.

The Darkest Dungeon, the game and the area, is an ocean of Demonic Spiders. Keep in mind however that preparing for them simply enables you to have a 'chance' not to be devastated. Not only does their very presence ruin your previously established 'best strategies' against an area's units, but they are each uniquely designed to synergize with the associated strengths and weaknesses of their area's normal units to overwhelm your party so not premptively preparing for their presence WILL devastate your team. Let it be known in general that ALL Veteran giant units and Champion elite units for each area and enemy type are this trope in the game.

If there was any doubt that Darkest Dungeon was not Nintendo Hard due to its prevalence of random chance, let these guys dispel it for you.

The Ancestor (notice he never said killing them would be easy)

Darkest dungeon cove champion party