"Crystal created by concentrating mana under a special environment. The crystallized form of rare mana.
Rare Prism is obtained by burning 4★ or 5★ Servants, obtaining extra Welfare servants, and from events that give Spiritron Dresses you already have.
With the Rare Prism Update, 4★ Servants yield 1 Rare Prism and 5★ Servants yield 5 rare prisms when burned.
Starting with the Merry Christmas from the Underworld (Rerun) you will get 1 Rare Prism for getting a sixth, and beyond, copy of a Welfare.
See Burning for more detailed information.