Theres a "boss" in Eternity Vault where you have to activate two pillar puzzles, after clearing the "encounter" and taking the loot, do not progress further.
If you go outside and reset the instance, then go in again, you can do it over and over again for more loot and technically gear up everyone
Tip; On Hardmode this encounter drops the Rataka pants which is Best in Slot.
Tip; To prevent this encounter bugging, always finish the South pillar before the North one.