Palace of Pain
As understood by the People transplanted on Installation 07, the Palace of Pain was a place of unspeakable horror where the Forerunners took humans and experimented on them. Few humans ever returned, and those who did were often "sick" and either died or were killed by the others as a result. However, some returned apparently unharmed.
In actuality, what the humans knew as the "Palace" was in fact the many Flood research facilities on the ring, controlled by the artificial intelligence Mendicant Bias under the supervision of the Primordial. In the facilities, they experimented on the effects of the Flood on humans, specifically the supposed immunity exhibited by some of them; however, this "immunity" turned out to be the result of the Flood's ability to choose whether to infect or not.
List of appearances