Scytode Dungeons are a more traditional style of dungeon, spawning as a boxed-in room made of Umbercobble and its variants. This makes them the sole source of Mossy and Webbed Umbercobble, as these cannot currently be crafted. They can be found in any Erebus biome.
These dungeons spawn in a similar manner to Overworld dungeons, but require more horizontal space, less vertical space, actually spawn with a roof (as normal dungeons do not), do not replace ores that spawn in their walls and require an opening on one part of their walls.
Inside a Scytode Dungeon, you will find a Web-covered Scytode spawner, along with up to 2 Chests filled with Erebus dungeon-tier loot.