A prismatic barrier is a personal magic defence much stronger than a magical shield in that it stops most attacks upon its user but what one can do whilst in one is severely limited. The Lasallian lyre artefact (or bottle of fairy dust) can raise a prismatic barrier but so can a few adventurer abilities, such as Groves Barrier or Terminus Kail.
Unlike a shield or an aura of weapons rebounding, prismatic barriers can be difficult for an attacker to penetrate or destroy. Other than direct attacks, barriers also block many damaging room effects, like Crystalism vibrations, Harmonics, and Devotion rites like Demons, though other powerful effects — holocaust globes, and Occultism Truename for instance — or the infliction of ongoing passive afflictions and room hinders are unaffected.