Some spells check a target’s magic resistance. Magic resistance is most often binary in nature, it either protects the target or it does not, with nothing in-between. Against some spells a successful resistance check merely means crippling injuries rather than instant death.
Some spells (mostly Astral) require the caster to defeat the target’s magic resistance before the spell can take effect. If a spell description states that “Magic resistance negates” then the following check is made:
Caster’s Penetration roll: 11 + Penetration bonuses + DRN + (additional skill in spell path) / 2 Rounding up.
Target’s MR roll: Magic Resistance + DRN + (skill in spell path) / 2 Rounding down.
The caster wins ties. Note that not all targets will have any magic skill, much less skill in that spell’s path, so often the target will simply add Magic Resistance and the DRN.
Magic boosts obtained from a communion are not considered in the above.
A notable exception to the above is the check to avoid death at the end of petrification: this always done at a roll of 13 + DRN vs MR + DRN + (earth skill) / 2.
If a spell description states that “Magic resistance negates easily” then the caster suffers a –4 modifier to his or her penetration roll. This is the same as "easy to resist." "Hard to resist" gives a +4 to penetration roll.
There was a bug in Dominions 4 that lead to communion masters losing penetration bonuses granted from items/bless. It looks like the bug might still exist in Dominions 5, tests seem to indicate -1 external penetration for 2 levels the caster misses (rounding down).