A team of 2 Webbers and 2 Spitters often results in one hero getting Stunned and Marked, followed by the whole group ganging up on them and quickly putting them to Death's Door. Web also inflicts a speed debuff, letting the Spiders act before the poor target can raise a finger to defend themselves. Both kinds also have Bite, an attack similar to Spit that also hits Marked targets harder, but it's considerably less powerful and reasonably inaccurate. The true demonic-ness comes when you have both together: Webbers can mark targets and stun them at the same time with Web, Spitters have a tremendously increased base damage and crit chance on Spit against a marked target note enough to bring the flimsier hero classes from full health to Death's Door on a crit. 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 RNG, let these guys clear that for you.

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

Darkest dungeon team for weald