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'Is there a reason why the primordial god of the underworld "Tartarus" doesn't currently make an appearance physically in Hades? You have Chaos, Nyx, and reference Gaia. Obviously Ouranos doesn't exist anymore per se.'

It's the same reason we haven't included the hundreds of other compelling characters from Greek myth. I guess there's another reason, which is: It's confusing to have both a location and a character. With Chaos, we decided to make an exception, as an expression of that type of idea from Greek myth.