Floating Ruins is a mod that adds floating islands with rooms that hold monsters and treasure to the world at a configurable spawn rate, and with configurable loot.
In The Epic Mod Pack, the spawn rate for floating ruins is decently high. I won't state the exact rate, but you should have no trouble finding one if you run around a bit. Finding one that other players haven't found first, however, is a bit more of a challenge.
As with much of the loot in The Epic Mod Pack, the chest rewards for the floating ruins have been configured specifically for the pack. You can find many items, of varying quality, in varying amounts from all of the various mods included in the pack. There are a few items obtainable from the floating ruins that are exclusively found in this way.