In LH 1.6, Mantle of Oceans Spell was changed:
https://forums.elementalgame.com/451979/page/1/#0
+ Fixed an issue causing Mantle of Oceans to reduce tactical spell costs by 60% instead of 40%.
MantleOfOceans Spell (LH 1.5):
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_TacticalSpellManaCost_Multiplier</StrVal>
<Duration>-1</Duration>
<Effect>E_MantleOfOceans_Particle</Effect>
<Value>-0.4</Value>
</GameModifier>
To this (LH 1.6, LH 1.7, and LH 1.8):
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_TacticalSpellManaCost_Multiplier</StrVal>
<Duration>-1</Duration>
<Effect>E_MantleOfOceans_Particle</Effect>
<Value>-0.6</Value>
</GameModifier>
This is what the Tactical Mana Cost UnitStat looks like with a variety of modifiers (LH 1.8):

I'd say the fix did exactly the reverse of what it was intending to do. That is, the new MantleOfOceans GameModifier value reduces Tactical Spell Mana Cost by 60% instead of 40%. This "fix" needs to be cancelled.
For comparison, the other GameModifiers are:
Archmage's Robe
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_TacticalSpellManaCost_Multiplier</StrVal>
<Value>-0.2</Value>
<Provides>-20% mana cost for tactical spells</Provides>
</GameModifier>
Mage (Rounded up to -0.3 on UnitStat Screen)
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_TacticalSpellManaCost_Multiplier</StrVal>
<Value>-0.25</Value>
<Provides>-25% mana cost for tactical spells</Provides>
</GameModifier>
Affinity
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_TacticalSpellManaCost_Multiplier</StrVal>
<Value>-0.1</Value>
<Provides>-10% mana cost for tactical spells</Provides>
</GameModifier>
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