One of those spell
s whose mere contemplation was a sign that your AD&D
(2nd edition) campaign was grossly overpowered.
The statistics are relatively straightforward: It's a nonreversable level nine spell belonging to the schools of abjuration and conjuration/summoning. Though its range is zero, its area of effect is a 10 foot radius; its duration is one turn per level of the caster. The casting of it requires only verbal components and seven time units.
The prismatic sphere demands a "special"-class saving throw.
So what does it do? To the spellcasting godling, this particular piece of magic is effectively a multipurpose tank / bomb shelter. It places the caster at the center of a seven-layered multicoloured (blinding creatures with fewer than 8 Hit Dice for 2D4 turns upon seeing it) sphere (unless the caster is in flight, it will appear as a dome above them with an unseen demisphere extending through the ground beneath them), each layer of which gives the wizard a distinct type of protection. The layers can be peeled back, one by one, by counterspells and each layer inflicts a punishment upon any being crossing its threshold without having neutralized it, also destroying any non-artifact or relic items being carried by the interloper in the deal.
- Layer One of the sphere, the outermost, is coloured red. Its function is to prevent the entry of nonmagical projectiles - arrows, crossbow bolts, sling shot. When crossed, it inflicts 20 Hit Points of damage, though the victim can make a saving throw to take "only" half damage. This layer is cancelled with the spell cone of cold.
- Layer Two of the sphere is of the orange persuasion. Its function is to prevent the entry of magical projectiles - magic missiles, Melf's acid arrows, and mundane arrows and such with enchantments on them. When crossed, it inflicts 40 Hit Points of damage (save for half) and the layer is negated by the spell gust of wind.
- Layer Three of the sphere is tinted a jaundiced yellow. It keeps out poisons, gases and agents of petrification. When crossed, it inflicts 80 Hit Points of damage (save for half) and the layer is stripped away by the spell disintegrate.
- Layer Four of the sphere is green. It stops breath weapons (from dragons, gorgons, cockatrices - we're not talking halitosis here. Even magic can't handle everything) and kills anyone crossing it unless they successfully make a saving throw vs. poison, in which case they lose a mere 20 HP. This layer melts to nothing when it meets a passwall.
- Layer Five of the sphere is coloured blue. It prevents the caster from being located, detected and protects them from mental (generally psionic) attacks. To successfully traverse this layer without negation requires a successful save vs. petrification (which is the result of an unsuccessful saving throw.) The blues pack their bags, however, if you throw a magic missile at it.
- Layer Six of the sphere is indigo. Not purple. Indigo. What does it do? It protects the caster from all magical spells cast towards them. What happens if you cross it without negating it? Well, Bob, you go insane. Unless, of course, you're fortunate enough to make a successful save vs. wand effects. To avoid the risk of losing your marbles in its passage, the uppity indigo layer can be knocked down with a lowly continual light enchantment.
- The Seventh, and innermost, layer of the prismatic sphere is a step to the left from the prior layer, a shade commonly known to us as violet. It confers the protection of the spell force field on the caster, and when someone goes through it, they may end up far, far away from the other side of the sphere - an unsuccessful saving through vs. spell effect will cast them into another plane. Wanna get rid of it? Finally something straightforward - dispel magic will peel back this final layer and let you get at the chewy wizard center within.
Don't have the patience or willingness to waste memorization time on a wimpy spell like continual light? Fear not! A prismatic sphere can also be foiled, all the layers together in one swell foop, with the application of a humble rod of cancellation
or the casting of Mordenkainen's disjunction