Thanks, Bahamut ZERO! The Ultimate DooMer, Stephen Clark in real, was a well-known modder in the Doom community. He made a lot of wads for Doom/Heretic/Hexen. When Serpent Ressurection was released he said that it was his last project for the community.
Also so since we’re talking about some fanmade goodness why not add some more?

Here's the good stuff I remember for Hexen:
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Caldera: (6 Levels)
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CHUXEN: 6 Levels also. Just beware of the traps in that one! ; )
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Ressurection of Chaos: A sequel to Hordes of Chaos. Somewhat a crossover between Heretic and Hexen.
link Note about Hordes of Chaos: Hordes of Chaos was a series of wads made for
Heretic actually, not Hexen! They are 5 of them, HOC I, II, III, IV, X. Here they are:
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Scourge of Viscerus: 7 Maps. Some musics taken straight out from Castlevania (Circle of the Moon, Symphony of the Night), assets from Doom 2. Some of these maps have genius level design, a must play.
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Shannara (5 maps):
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Starship (6 levels) :
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Wolfen (6 maps):
link