In fairness on that one, Mordenkainen was Greyhawk too. They tend to just mix their IP's nowadays.Flipping through Tasha's Cauldron of Everything I was surprised to see art depicting Rhys given that Tasha is from Greyhawk and a lot of the fluff of the book revolves around her. If I had a more conspiratorial spirit I might think that this is a hint that Planescape is coming soon, but really I think that this is just an easter egg reference.