The patch notes from July 31st indicate that the new underworld heritages will be buyable playing a neutral faction, but they still are not. I was able to buy the underworld diplomacy upgrade, which unlocked the dwarf and drow coins, but no option to purchase the dwarven heritage.
In order to purchase Dwarves and Drow heritages, you must meet these criteria: unlocked Drow/Dwarves Champion trophy (buy all faction upgrades, required to unlock any kind of heritage), purchased Underworld Diplomacy (required to unlock their faction coins), collect enough faction coins. If you have fulfilled all the requirements but are still unable to purchase the heritage, please leave us a PM.
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In order to purchase Dwarves and Drow heritages, you must meet these criteria: unlocked Drow/Dwarves Champion trophy (buy all faction upgrades, required to unlock any kind of heritage), purchased Underworld Diplomacy (required to unlock their faction coins), collect enough faction coins. If you have fulfilled all the requirements but are still unable to purchase the heritage, please leave us a PM.