Viconia has 65 Magic Resistance. What does this mean? What is and isn't she resistant to and how is this worked out?

How does the Lower Resistance spell work?

How is magic resistance different from Save vs Spells?

Magic resistance is the percentage chance that magic will fail to affect a character completely and works in addition to a saving throw. If a spell is cast against Viconia, she has a 65% chance for the spell to fail completely. If it does not, she will still get a saving throw (if applicable for the spell).

Magic resistance is not proof against all magic though--some spells are designed to specifically bypass it either by design (Lower Resistance or beneficial spells such as Heal for example) or by bug.

Lower Resistance lowers the percentage for a period of time. So if Viconia is hit with a Lower Resistance, it'll lower that 65% for a short period of time.