Rare Prisms are a rare currency that can be exchanged at Da Vinci's Workshop for various items, Craft Essences or Mystic Codes.
The primary way to obtain Rare Prism is by burning a 4★ or 5★ Servant in the Da Vinci's Workshop, or by using a 4★ or 5★ servant as an EXP fodder.
(Non-summoned servants will not yield any Rare Prism after burning or being turned into EXP)
A Rare Prism