I believe maxvalue minvalue only works for damage modifiers. You'll have to simulate it with a non-linear function. Actually I've never tried if ^ is a valid operator.

How exactly are you casting the spell? If it is coming from a trait, ie battleautocastspell in battle, you can work with 3 spells instead of 1 and have them trigger only if 1, 2 or 3 death shards are available. Then you'd have a trait that gives you 10%, 20% or 30% in battle.

It basically comes down to "encyclopedical" knowledge of the XML and how the game interprets it, that you get from modding the game and its predecessors for a long time. No joke, I probably spent 1000+ hours modding Elemental games.

That, and it just so happens that someone else probably tried to do something similar at one point.

This is an effect I have in Children of Storm: Angels get +1 Moves in battle if the faction controls 3 or more life shards. In the bloodline I have

      <GameModifier>
        <ModType>Unit</ModType>
        <Attribute>BattleAutoCastSpell</Attribute>
        <Provides>.</Provides>
        <StrVal>Ef_NewFaith2</StrVal>
        <Value>100</Value>
      </GameModifier>

and the spell looks like this

  <SpellDef InternalName="EF_NewFaith2">
    <DisplayName>New Faith: Moves</DisplayName>
    <Description>+1 Moves for controlling 3 Life shards.</Description>
    <IconFG>Icon_ManaToMaintain.png</IconFG>
    <IconColor>210,110,210</IconColor>
    <CanStack>0</CanStack>
    <SpellBookSortCategory>Unit</SpellBookSortCategory>
    <SpellBookSortSubCategory>UnitEnchantment</SpellBookSortSubCategory>
    <SpellType>Tactical</SpellType>
    <SpellClass>Defensive</SpellClass>
    <SpellSubClass>Buff</SpellSubClass>
    <SpellTargetType>Self</SpellTargetType>
    <HideInHiergamenon>1</HideInHiergamenon>
    <IgnoreCastAnim>1</IgnoreCastAnim>
    <IsCastable>0</IsCastable>
    <LifeShards>3</LifeShards>
    <GameModifier>
      <ModType>Unit</ModType>
      <Attribute>AdjustUnitStat</Attribute>
      <StrVal>UnitStat_Moves</StrVal>
      <Duration>-1</Duration>
      <Value>1</Value>
    </GameModifier>
    <HitSoundFX>Spell_Enchant_01</HitSoundFX>
  </SpellDef>

So you could easily make copies of this and give it reflect damage instead, making it "10% per death shard up to a max of 30%"

That heavenfall... was that something in WOM or a good programming guess

Ahhh.. well lets  continue our boxing with one arm and leg