What are some D&D products that used a 3rd-party's IP?

[Paul Farquhar]

pretty sure Lankmar, aka the Nehwon Mythos, was in the original Deities and Demigods

It was.

 

[QuentinGeorge]

The inclusion of Cthulhu turned out problematic for TSR, for while it was a natural inclusion for D&D, apparently it had already been licensed out. Kind of a shame, because I think it would have made for a better horror themed setting than Ravenloft did. I wish that Lankmar would have continued, and never really found out why that setting died off.

Oh, and as much as I love Rokugan, technically it was WotC IP when it was the OA in 3E. This was during the timeframe that WotC bought it from AEG, before they sold it back to them years later (I think AEG went through a bankruptcy or something to restructure). This was a good thing IMO, because the 3E system doesn't provide the nuance necessary to run such a setting properly, where social interaction is more important than combat. I think that 5E could do it better (mostly due to bounded accuracy), but it would still have difficulty in conversion (shugenja for example, would probably have to be its own class as none of the spellcasting classes really work).

There was even more Lovecraft baked in that just didn't get explicitly mentioned. The Maure family's patron "Y" is Yog-Sothoth, and it is all but openly stated at one point that Graz'zt's unnamed father (Pale Night is his mother) is Nyarlthathotep. (Makes sense one you look at Graz'zt)

 

[Umbran]

pretty sure Lankmar, aka the Nehwon Mythos, was in the original Deities and Demigods

IIRC, it was in the first printing(s) of the book, but was removed in later printings due to some licensing confusions that they ironed out for later products.

 

[Mookus]

TSR's "Alice In Wonderland" duo is the first I recall (not already mentioned, unless I missed it):

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