Despite the Ancient Gateway being added in the Heart of the Ruins (part 1) update it wasn't possible to regenerate the ruins as the Ancient Gateway had Black and Yellow tapes all over it, on the Heart of the Ruins (part 3) update the Odd Skeleton was able to be reanimated next to the Ancient Gateway in the Atrium, triggering the boss fight.
It is possible to trigger this event by running the "c_resetruins()" command on the console interface.