Does that only protect against spells or is it immune to everything (ex: fire, lightning, cold damage)?
From the Core Abilitiles:
<AbilityBonus InternalName="ImmuneToMagicAbility">
<AbilityBonusOption InternalName="ImmuneToMagic">
<DisplayName>Immune to Magic</DisplayName>
<Description>Immune to Magic</Description>
<Icon>Ability_ImmuneToMagic_Icon.png</Icon>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>ProtectFromSpells</Attribute>
<Provides>Immune to Magic</Provides>
</GameModifier>
<CombatRating>50</CombatRating>
<IsCombatAbility>1</IsCombatAbility>
<Type>Defensive</Type>
<AIData AIPersonality="AI_General">
<AIPriority>5</AIPriority>
</AIData>
</AbilityBonusOption>
</AbilityBonus>