Reduces all damage taken by Y% and reduces the fade time of your Invisibility spell by X sec.
Prismatic Cloak is a Mage talent in the Arcane tree that reduces all damage taken by 2% per rank, up to 6% at max rank. This talent applies to all damage, magic and physical, making it a nice survivability talent.
Its most notable effect however is the effect it has on Invisibility, turning the spell instant at maximum rank which is a massive increase to Arcane survivability in PvP settings. For this, it is considered a required talent in any serious Arcane PvP build.