The Ruins is a naturally generated biome introduced with the Tremor mod. The biome is a large Stone Brick structure with Water Candles strewn about. Within the biome there are Ruin Altars, naturally generated structures that are used to summon the Ancient Dragon. Ruin Ghosts will spawn randomly as well. There are also Ruin Chests which when unlocked contain a variety of loot, similar to that of the Golden Chests in the Dungeon.
There can be multiple Ruins generated within a world at a time. They only generate underground within the Cavern layer.