Alright I tried making a player ability Ancient Legacy:
<AbilityBonus InternalName="Ancient_LegacyAbility">
<AbilityBonusType>Player</AbilityBonusType>
<AbilityBonusOption InternalName="Ancient Legacy">
<DisplayName>Ancient Legacy</DisplayName>
<Description>Grants access to a unique set Buildings and abilities.</Description>
<Icon>Ability_Archers_Icon.png</Icon>
<GameModifier>
<Provides>Grants access to a unique set Buildings and abilities.</Provides>
</GameModifier>
<Cost>0</Cost>
<Type>Army</Type>
<AIData AIPersonality="AI_General">
<AIPriority>5</AIPriority>
</AIData>
</AbilityBonusOption>
</AbilityBonus>
Added it to the race xml file:
<SelAbilityBonusOption>Ancient Legacy</SelAbilityBonusOption>
And modified the building:
<ImprovementType InternalName="Mana Forge">
<DisplayName>Mana Forge</DisplayName>
<Description>Mana Forges increase the yields of any city they are built in.</Description>
<RequiresCity>1</RequiresCity>
<BarredTerrain>River</BarredTerrain>
<BarredTerrain>SwampTerrain</BarredTerrain>
<PreferredTerrain>City</PreferredTerrain>
<PreferredTerrain>Category:Land</PreferredTerrain>
<PreferredTerrain>Forest</PreferredTerrain>
<Prereq>
<Type>AbilityBonusOption</Type>
<Attribute>Ancient Legacy</Attribute>
<Target>Player</Target>
</Prereq>
<LaborToBuild>120</LaborToBuild>
<GameModifier>
<ModType>Resource</ModType>
<Attribute>TileYieldEssence</Attribute>
<Value>1</Value>
<PerTurn>1</PerTurn>
<Provides>+1 Essence</Provides>
</GameModifier>
<AIData AIPersonality="AI_General">
<AIPriority>20</AIPriority>
<AITag>Study</AITag>
</AIData>
<ArtDef>Art_ScryingPool</ArtDef>
</ImprovementType>
I tried starting a new game and the Mana Forge showed up again in the Sovereign screen however when I started a new game it still was not showing up in the build queue.