I've noted earlier that the Maul mechanic is actually semi-moddable: https://forums.elementalgame.com/417078
As it turns out, the Auto-defend that occurs when a unit "passes" his action without doing anything is also moddable. In CoreSpells.xml we find
<SpellDef InternalName="AutoDefend">
<DisplayName>Defending</DisplayName>
<Description>When a unit passes their attack action, their Defense is automatically doubled until their next action.</Description>
<IconFG>Ability_Defend_Icon.png</IconFG>
<IconColor>32,45,243</IconColor>
<SpellType>Tactical</SpellType>
<SpellClass>Defensive</SpellClass>
<SpellSubClass>Buff</SpellSubClass>
<SpellTargetType>Self</SpellTargetType>
<HideInHiergamenon>1</HideInHiergamenon>
<IsCastable>0</IsCastable>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Defense_Blunt</StrVal>
<Duration>1</Duration>
<UpdateDurationAtTurnStart>1</UpdateDurationAtTurnStart>
<Effect>Y_Defend_Particle</Effect>
<Calculate InternalName="Calc" ValueOwner="CastingUnit">
<Expression><![CDATA[[UnitStat_Tactical_Auto_Defense_Boost]]]></Expression>
</Calculate>
<Calculate InternalName="Value">
<Expression><![CDATA[[Calc]]]></Expression>
</Calculate>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Defense_Cutting</StrVal>
<Duration>1</Duration>
<UpdateDurationAtTurnStart>1</UpdateDurationAtTurnStart>
<Calculate InternalName="Calc" ValueOwner="CastingUnit">
<Expression><![CDATA[[UnitStat_Tactical_Auto_Defense_Boost]]]></Expression>
</Calculate>
<Calculate InternalName="Value">
<Expression><![CDATA[[Calc]]]></Expression>
</Calculate>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Defense_Pierce</StrVal>
<Duration>1</Duration>
<UpdateDurationAtTurnStart>1</UpdateDurationAtTurnStart>
<Calculate InternalName="Calc" ValueOwner="CastingUnit">
<Expression><![CDATA[[UnitStat_Tactical_Auto_Defense_Boost]]]></Expression>
</Calculate>
<Calculate InternalName="Value">
<Expression><![CDATA[[Calc]]]></Expression>
</Calculate>
</GameModifier>
</SpellDef>
I've bolded the stat which the ability draws its strength from - UnitStat_Tactical_Auto_Defense_Boost. The value is by default 5, and can be boosted by Defender traits and shields of various types.
Knowing the spell is moddable allows us to put new stuff in there. For example, currently the Auto-defend only boosts physical defenses. We could add effects that also increase spell resist or magical attack resists (fire, cold, lightning, poison, arcane). Or we could do other things like have a unit heal if it passes the turn. A melee monk that gains dodge instead. A wizard that prepares the next spell by reducing its mana cost. Of course it would all have to be inside the same SpellDef but it's doable.