Prism may refer to several things. First, there are spell prisms, items used for Multicast Spells. These are dropped (rarely) in thrasher chests and in Brax. A one-sided prism is located in Brax but it requires an Ao3 to access.
Second are the elemental prisms in the Desert. These are end-game "raid" zones typically requiring a group of players working together in order to kill the prism guardians. In addition to excellent treasure, defeating each prism has a temporary effect in the desert and a permanent increase in associated resistances (at least until reincarnation). The first victory increases respective base resistance by 5%, maximum by 2%, and increases damage by 5%. Once cleared, the prisms remain so for 72 hours.