i just took a look at the XML for some of the bows I was experimenting with and it refreshed my memory somewhat. Below is one I got to work though I don't recall for sure how well it worked. It supposedly did pierce and cold attack but I'm thinking it didn't do but one of them. (Sorry, the memory is kinda foggy as I never used this in an actual game) I think the issue has to do with the bow spell using that <Attribute>DefendableDamage</Attribute> tag. The damage the bow attack does is passed to the spell (based on the attack rating of the bow) and that process seems to block the spell from doing any other damage. Like I said, I was able to get other, non-damage spells to work. In this case, the cold damage had a "chilling" effect on your agility and cast a "Slow" on the target. Perhaps if you got rid of the damage being passed to the arrow spell, it might open up damage from within the spell. Maybe getting rid of the <IsRangedAttack> will help. I don't remember if I ever tried that or not.
<GameItemType InternalName="AIM_CalaQuessirLongbowBoreal">
<DisplayName>Cala'Quessir Boreal Longbow</DisplayName>
<Description>Fire fast, the arrows start to freeze when the bow is drawn. </Description>
<Type>Weapon</Type>
<Type>Defense</Type>
<WeaponType>Bow</WeaponType>
<CanBeEquipped>1</CanBeEquipped>
<ShopValue>1720</ShopValue>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Accuracy</StrVal>
<Value>4</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Attack_Pierce</StrVal>
<Value>10</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Attack_Cold</StrVal>
<Value>1</Value>
<PerLevel>1</PerLevel>
<Provides>+1 Cold Attack per Level</Provides>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_CurrentWeight</StrVal>
<Value>8</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>UnlockRangedAction</Attribute>
<StrVal>AIM_BowAttack_Freezing</StrVal>
<Provides>Ranged attack</Provides>
</GameModifier>
<IsAvailableForSovereignCustomization>0</IsAvailableForSovereignCustomization>
<Likelihood>250</Likelihood>
<RarityDisplay>UltraRare</RarityDisplay>
<TacticalRange>8</TacticalRange>
<HeroOnly>1</HeroOnly>
<IsAvailableForUnitDesign>0</IsAvailableForUnitDesign>
<Prereq>
<Type>Race</Type>
<Attribute>Race_Type_CalaQuessirs</Attribute>
</Prereq>
<Prereq>
<Type>Tech</Type>
<Attribute>AIM_CalaQuessir_Arcane_Bows</Attribute>
</Prereq>
<Prereq>
<Type>UnitStat_Level</Type>
<Attribute>9</Attribute>
</Prereq>
<AIData AIPersonality="AI_General">
<AIPrefType>AIPrefType_BOW</AIPrefType>
<AIPriority>40</AIPriority>
</AIData>
<ArtDef>AIM_Longbow_Freezing_ArtDef</ArtDef>
<GameItemTypeArtDef InternalName="AIM_Longbow_Freezing_ArtDef">
<GameItemTypeModelPack InternalName="AIM_Longbow_Freezing_Default">
<IconFile>W_Longbow_Yew_Masterwork_Icon_01.png</IconFile>
<TintR>0</TintR>
<TintG>0</TintG>
<TintB>0</TintB>
<AttackSFX>Hit_Arrow1</AttackSFX>
<AttackSFX>Hit_Arrow2</AttackSFX>
<EquipSFX>Equip_BowandArrow_01</EquipSFX>
<EquipSFX>Equip_BowandArrow_02</EquipSFX>
<EquipSFX>Equip_BowandArrow_03</EquipSFX>
<SoundPack>SoundPack_Bow</SoundPack>
<GameItemTypeModel>
<ModelFile>Gfx\HKB\Weapons\W_Longbow_Yew_Perfect_01.hkb</ModelFile>
<Texture_All>Gfx\HKB\Armor\FE_Weapons_02.png</Texture_All>
<Attachment>hand_Left_Lcf</Attachment>
<Color_All>232,158,28</Color_All>
</GameItemTypeModel>
<GameItemTypeModel>
<ModelFile>Gfx\HKB\Armor\K_ArcherQuiver_01.hkb</ModelFile>
<Texture_All>Gfx\HKB\Armor\K_Female_Armor_Archer_Texture_01.dds</Texture_All>
<Attachment>back_Lcf</Attachment>
</GameItemTypeModel>
</GameItemTypeModelPack>
</GameItemTypeArtDef>
</GameItemType>
<SpellDef InternalName="AIM_BowAttack_Freezing">
<DisplayName>Bow Attack Ice</DisplayName>
<Description>Attack a unit from a distance with a single arrow from your bow.</Description>
<Image>Action_BowArrow.png</Image>
<IconFG>Action_BowArrow.png</IconFG>
<IconBG>Action_BowArrow.png</IconBG>
<IconColor>255,255,255</IconColor>
<SpellType>Tactical</SpellType>
<SpellClass>Offensive</SpellClass>
<SpellSubClass>Damage</SpellSubClass>
<SpellTargetType>EnemyUnit</SpellTargetType>
<HideInHiergamenon>1</HideInHiergamenon>
<IsCastable>0</IsCastable>
<IsRangedAttack>1</IsRangedAttack>
<IsResistable>1</IsResistable>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>DefendableDamage</Attribute>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_CombatSpeed</StrVal>
<DisplayName>Slowed</DisplayName>
<Duration>-1</Duration>
<Effect>E_Slow_Particle</Effect>
<Calculate InternalName="Calc" ValueOwner="CastingUnit">
<Expression><![CDATA[[UnitOwner_GetNumWaterShards] * -2]]></Expression>
</Calculate>
<Calculate InternalName="Value">
<Expression><![CDATA[[Calc] - 4]]></Expression>
</Calculate>
</GameModifier>
<AIData AIPersonality="AI_General">
<AIPriority>5</AIPriority>
</AIData>
<HitSoundFX>Hit_Arrow1</HitSoundFX>
<HitSoundFX>Hit_Arrow2</HitSoundFX>
<MissSoundFX>Miss_Swing2</MissSoundFX>
<SpellCastEffectName>AIM_FrostLongbow_Particle</SpellCastEffectName>
<SpellCastEffectScale>0.3</SpellCastEffectScale>
<SpellCastProjectile>1</SpellCastProjectile>
<SpellCastProjectileSpeed>700</SpellCastProjectileSpeed>
<SpellDefEffect>
<EffectName>AIM_FrostLongbowImpact_Particle.xml</EffectName>
<LocalPosition>0,0,0</LocalPosition>
<EffectScale>1</EffectScale>
<EffectDelay>0</EffectDelay>
<SnapToTerrain>1</SnapToTerrain>
</SpellDefEffect>
</SpellDef>