It is based on the spell being cast by a M-U of 11th level,
and it must be adjusted upwards by 5% for each level
below 11th or downwards for each level above 11th of the
M-U casting the spell.

Thus a MR of
95% means that a Necromancer (MU10) has no possibility
of affecting the monster with a spell, while a Wizard (MU12) has a 10% chance.

Even if a spell does take effect on a magic-resistant creature, the creature is entitled to normal saves.
Note also that the MR of a creature has an effect on certain existing spells such as hold portal, where it indicates the probability of the MR shattering the existing spell.

DEITIES AND DEMIGODS Version
MAGIC RESISTANCE: This indicates the percentage chance of any spell absolutely failing to affect the magic-resistant creature. It is based on the
spell being cast by a spell caster of 11th level, and it must be adjusted upwards
5% for each level below 11th or downwards 5% for each level
above 11th of the spellcaster casting the spell.
Thus a magic resistance of 95% means that a 10th level spell-caster has no chance of affecting the creature with a spell or spell-like effect,
while a 12th level spell-caster has a 10% chance.
Even if a spell does affect a magic-resistant creature,
the creature is entitled to normal saving throws (if applicable).
 A creature's magic resistance extends only to its immediate possessions, i.e. anything carried or won.
Area-effect spells will still function if targeted with a magic-resistant creature within their area;
the creature itself might not be affected,
though all others in the spell area will be subject to spell effects.
(A fireball, for example, may wipe out a cluster of orcs,
and a devil standing in their midst might be totally unaffected due to its magic resistance).
The magic resistance of a creature has an effect on certain existing spells such as hold portal,
where it indicates the probability of the magic resistance shattering the existing spell.)

Special Note: Certain deities have special abilities (of a magical origin)
that "never fail", e.g. a weapon that never misses, or an ability against
which there is no save. Creatures with magic resistance may still avoid the
effects of these abilities if they make a successful magic resistance roll. To
determine level of the deity with the magic ability, use the deity's highest
level character class ability, unless it is less than 15th level for demigods,
20th level for lesser gods, or 25th level for greater gods, in which case the
aforementioned levels should be used.

To determine level of the deity with the magic ability, use the deity's highest level character class ability,
unless it is less than 15th level for demigods, 20th level for lesser gods, or 25th level for greater gods, in which case the aforementioned levels should be used.

FIEND FOLIO Version
MAGIC RESISTANCE indicates the percentage chance of a spell
absolutely failing if it is cast at, or on, the monster in question. The
basisofthefigureisaspellcast byamagic-userofthellthexperience
level; the figure must be adjusted upwards by 5% for each experience
level below 11 th, or downwards by 5% for each experience level above
l l t h , of the spell-caster. Thus, a monster with 95% magic resistance
cannot be affected by a spell cast by a magic-user of the 10th experience
level or lower, while a 12th level magic-user has a 10% chance of
affecting it.
Note that, even if a spell does “overcome the magic resistance” of a
monster, the monster is still entitled to normal saving throws.

MONSTER MANUAL II Version
MAGIC RESISTANCE is the chance a spell might fail when cast on the
monster; this chance is expressed in a percentage. The basis for the
percentage isa spell cast by a magic-user of the 11th experience level. If
player character spell casters are not of the 11th level, the figure must
be adjusted upwards by 5% for each level below 11 th, or downwards by
5% for each level above 11th. Thus, a monster with 95% magic resistance
cannot be affected by a spell cast by a magic-user of the 10th
experience level or lower, while a 12th level magic-user has a 10%
chance of affecting it.

Note that even if a spell does overcome the magic resistance of a
monster, the monster is still entitled to normal saving throws. A creature's
magic resistance extends only to its immediate possessions, i.e.,
anything carried or worn. Area-effect spells will still function if targeted
on a magic-resistant creature within their area. The creature itself
might not be affected, although all others in the spell area will be subject
to spell effects. A fireball, for example, may wipe out a cluster of orcs,
while an agathion standing in their midst might be totally unaffected.
The percent of magic resistance of a creature has an effect on certain
existing spells such as hold portal, where it indicates the probability of
shattering the existing spell.

Thanks to David Folger for his insightful comments!

ADQ: How would a creature's magic
resistance affect area-effect spells or
magical walls (stone, ice, etc.)?

ADA: Against area effects (such as Fireball),
the MR is checked at that point
when the creature might be affected. If
the MR destroys such a spell effect, it
cancels it only around the creature's
body, not protecting others nearby. If
the creature approaches a magical wall,
MR is checked when the creature
touches the wall. If successful, the wall
disappears. If not, the creature may
touch it every round, causing an MR
check every round until the creature
gives up or the wall goes down. If the
creature touches (hits) a character upon
whom spell effects exist (e.g. a mage
with Detect Invisible, Polymorph, Protection
from Evil, and Detect Magic
running), each spell effect must be
checked separately to see what crumbles
and what remains.
(Polyhedron #12)

ADQ: Is magick resistance cumulative for
the various items which grant it to the
possessor, such as a robe of the arch-magi
and a pseudo-dragon, a paladin with 2
holy avengers, etc.? If so, then what is
the maximum amount of magick resistance?
ADA: Magick resistance is cumulative for
multiple items. Quite bluntly, however, if
holy swords are common enough that 2
can be carried by a single paladin in your
campaign, you are so far outside a normal
game's rules that you are free to ignore
this one as well.
(Polyhedron #22)
 
 

Question: If a creature has MR and someone is using a magical
weapon against it, does their MR affect the USE of these kinds of
weapons (fail to work), or do they work
normally?

Answer: As stated in the Explanatory
Notes in the front of the MM,
“Magic resistance indicates the percentage chance of any spell absolutely failing.”
Magic resistance does not have
any effect on a creature’s ability to with-
stand damage from magickal weapons or
any magickal item which can damage or
otherwise affect the creature, unless the
description of that weapon or magic
item specifically states otherwise.
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Question: If an erinyes (30% MR) is out to get a 27th-level M-U,
and the character casts an attack spell,
would the erinyes make a MR roll at a minus?

Answer: No.
Rolls for MR are never made “at a minus.” As
defined in the explanation of MR in the front of the
MM; any spell cast against an erinyes by an 11th-
level character has a 30% chance of absolutely failing. For each
level of the spell-caster above 11th, the chance of spell failure
decreases by 5%, which means that a 17th-level spell-caster (or
one of any higher level) will automatically be able to negate the
MR of the erinyes.
The lowest effective MR that a creature can ever have is 0%, not a minus number.

Mialee fails to overcome the <magick> resistance of <Marilith> [sic]

Bombay wrote:
Situation #2
A Demon with MR% attacks a cleric who has CAST Protection from Evil 10r, does the demon get a check for MR%
The basic question we have been mulling over is, Is the intent of MR% only spells that are CAST at you, or any spells that are currently in effect?(Pro Evil or Darkness?)
 

#2: The MR of any NPC/monster applies only to magic directed at the person of the one possessing MR.
Protections, and magic weapons will not be affected my MR, nor will things the possessor encounters be checked for that effect.
 

garhkal wrote:
Gary. What wall spells are supposed to be affected by magic resistance?
Ie, if i cast a wall of iron over a drow, would the drow get squished, or would his MR kick in>
What about rock to mud under their feet???
 

Ciao!

As a matter of fact I did not allow a wall spell to be cast save if there was some surface the bottom edge it could rest upon.

If you allow them to be cast into the blue, then MR will not affect one in falling, as it is not a spell.
MR does not affect blosw from magic wea[ons, eh?
The same goes for rock turned to mud.

Cheers,
Gary