Spell Resistance (SR) is a special anti-spell defense possessed by some creatures. Spell Resistance is an additional layer of defense against most non-damaging magical attacks, in that it adds an extra roll that the caster has to make for his magic to "land" on that target.
Spell Resistance only matters if the spell's creature actually has it, and the spell being cast is one that can be stopped by Spell Resistance. In DDO, spells that most purely cause damage ignore Spell Resistance.
The most common creatures with Spell Resistance will be Drow, Duergar, Devils, anything with "Fiendish" in the name and some bosses. (Anticipate spell Resistance to be at least CR plus 10 for regular monsters. Red and purple named bosses tend to have a much higher CR, and generally have spell resistance within 0-5 points of their CR.)
Caster Level check do not automatically succeed on a 20, neither automatically fail on a 1.
Spell Resistance on player characters
Player characters can gain spell resistance as an alternative / in addition to having high saving throws. SR is particularly useful against spells and abilities which don't offer a saving throw, such as gaze of umber hulks or many beholder ray attacks.
Sources of SR:
- Monk's Diamond Soul class feature
- Deep Gnome's / Drow's racial spell resistance
- Spell Resistance (spell)
- Spell Resistance, Mass spell
- Occult Slayer's capstone enhancement (stacks with racial SR, does not stack with items)
- Ward against Weird (stacks with Insightful SR on items)
The above sources don't stack. They also don't stack with items.
Items can grant bonus to SR:
- Enhancement bonus - available on named items, random loot and via cannith crafting
- Insight bonus - available on random loot and via cannith crafting
- Quality bonus - available on named items
Additional stacking sources:
- Spellshield Aura: +6 sacred bonus
- Grandmaster of Flowers: +1 per core
- Serenity: +9 twistable (tier 2)
Monster spell penetration roll is usually 1d20 + CR. Therefore, having SR above monster CR+20 should make you protect you against all spells and effects with SR check.