Spread the light, my child.
Henzell says...
  1. Willpower provides the ability to resist certain hostile enchantments that attempt to overpower the caster's strength of will. Player Will comes from XL, Trog's hand, and items. The most important hostile enchantments that attack willpower are confuse, slow, paralysis, and banishment.
  2. Chances of anything resisting a spell that attacks willpower are calculated as follows: (willpower + 100 - the spell's power) is compared to ( random2(100) + random2(101) ), and if the random numbers are less the spell is successfully resisted.
  3. Monster willpower is a property of the monster type and HD. Player willpower is XL * racial-multiplier + 40 per item, mutation level, or artefact + of Will, or for being in lichform. Racial factor is 7 for Sp, 6 for purple Dr, Fe and DD, 5 for Na, Mu and VS, 4 for DE, Dg, Vp, Fo and Og, and 3 for everyone else.
  4. Called "magic resistance" before 0.26 and commonly referred to as MR.
  5. <minqmay> what "hostile enchantments" really means is "effects that can be resisted by "resistance to hostile enchantments""

Information based on master branch and the great LearnDB.
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