Empire of Zul or Zul Empire is basicall Zandalari Empire which exist in Zandalar. It is a lengthy read so give time to check wiki entry out.
When I specifically say Zandalari I mean Zul empire not inhabitants of Zandalar Isle so, Sethrek race are disregarded since they don't belong to the Troll race and specifially Zandalar empire.
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Empire of Zul or Zul Empire is basicall Zandalari Empire which exist in Zandalar. It is a lengthy read so give time to check wiki entry out.
Although it is said to have a defunct status. Therw was already a story progression happening already starting with 4.0 with Rise of Zul as well as the Thunder King Patch. The reason why Rhastakhan let Zul set sail on expedition is to notify the splintered smaller empires to unite and meet up once again.
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Do present any bluepost, official press release, In-game text entries, existing publications that directly says she was never a Dark Troll. If you cannot present any then I would take it as mere presumption and hypothesis.