The higher a being's MR, the less likely that it will be affected by an enchantment cast on it. Beings that are magical in some way (such as elves or sirens), magic-users, and those that are non-natural tend to have higher MR than other creatures.
Monster MR is partly based on hit dice, so monsters with high hit dice tend to be highly resistant to magic. Most monsters with one hit die have a 50% chance to fail to resist outright, in addition to the normal saving throw. Finally, some monsters are outright immune to magical enchantments - for instance, most nonliving creatures.
Some of these effects can come from potions, weapon brands, clouds, or spells that are not considered enchantments.
- Brain Feed
- Cause Fear
- Dimension Anchor
- The teleportation effect of the Dispersal spell
- Ensorcelled Hibernation
- Inner Flame
- Mass Confusion
- Scroll of fear
- Sentinel's Mark
- Sleep (but not the effects of Dream Dust from dream sheep)
- Strip Resistance (ignores MR 33% of the time)
- Teleport Other
- Wand of disintegration
- Wand of enslavement
- Wand of paralysis
- Wand of polymorph
- Many of these enchantments can be resisted via other means. For example, undead, nonliving, and plant monsters cannot be slept.
- Polymorph will not be resisted by (very) ugly things, which will always change color. Shapeshifters can only resist it if they are currently in the form of something immune to magic.
- Effects produced by the chaos brand don't check MR.
Resisting an enchantment is dependent on the resisting creature's MR and the enchantment power (which is a function of the spell power) of the enchantment it is resisting. The formula is:
If 1d100 + 1d101 is less than 102 + MR - the enchantment's power, the enchantment is resisted.
This is sometimes referred to as a "saving throw", after the die-based resistance system employed in Dungeons & Dragons.
Enchantment power is your spell's power multiplied by a modifier (usually 1) and then passed through a stepdown function which halves your spell power every 30 points above 40. Agony, Pain, Slow, Corona, Teleport Other and all the mass enchantments have modifiers greater than 1. Ensorcelled Hibernation has an additional stepdown.
Neither the player's nor monster's MR is ever given explicitly, but a rough suggestion of how magic resistant they are can be seen by pressing the % key for the player, and examining a monster's description. The specific number can be calculated, however.
Pluses on these screens correspond to the MR level of the player or monster, as follows:
|Pluses shown||Player MR||Monster MR|
|+++++||200 or more||161 or more|
As a practical matter, a player who has MR++++ should be able to usually resist the most dangerous enchanters in the game (such as deep elf demonologists, liches and ancient liches). More MR will reduce the likelihood of these spells landing even further, but the gains become significantly less important beyond this point.
Player resistance is determined as below:
Experience level × racial multiplier, plus the following:
- Shadow Form: Immune
- Randart MR (40 for each +)
- 40 for each piece of armor of magic resistance
- 40 for each ring of protection from magic
- 40 for each rank of the MR mutation
- 40 in lichform
- 80 when using Trog's Hand
- Everything is then divided by two if the player is under the effect of a scroll of vulnerability, the attack of a phantasmal warrior, or the Strip Resistance spell.
Racial multipliers are as follows:
|MR per XL||Species|
|6||Deep dwarf, Felid, Purple draconian|
|5||Naga, Vine stalker, Mummy|
|4||Deep elf, Demigod, Vampire, Formicid, Ogre|
|3||All other species|