Revelations- what is this?

[rhodo]

I've found several of these in Revelations. What are they? And I'll be dipped if I can find the spawners. What am I missing? (Plains biome loot rooms with thatch roofs.)

Thansks! (And nice mod pack after a few years not playing Minecraft...)

https://imgur.com/a/IoiYl4Z

 

[Citron]

It's a little building by Roots2. There's an invisible spawner inside (it's an entity, not a block) that spawns skeletons and zombies.

 

[rhodo]

Thank you! I'll check out that mod.