If anyone's looking for some official artwork for the series, you can find some pieces here:

Heretic weapons:
https://www.instagram.com/p/2Y4GEuqmMv/

D'Sparil from Heretic:
https://www.instagram.com/p/2Y4APkqmMe/

D'Sparil from the Heretic 2 manual:
https://www.instagram.com/p/2Y4ETbqmMo/

Concept art of Corvus, the main character of Heretic 2:
https://www.instagram.com/p/2Y4Bw2KmMi/
https://www.instagram.com/p/2ULmSPKmNs/

Heretic cover by Brom:
https://imgur.com/udnnVJG
In addition to providing the cover art, Gerald Brom contributed several character/monster designs to Heretic 2.

Heretic Shadow of the Serpent Riders box art:
https://www.mobygames.com/images/covers/l/3553-heretic-shadow-of-the-serpent-riders-dos-front-cover.jpg

Hexen box art:
https://www.mobygames.com/images/promo/l/636920-hexen-beyond-heretic-other.jpg
Deathking of the Dark Citadel:
https://www.mobygames.com/images/promo/original/1597938055-2629011001.jpg

Hexen 2 promo art from the Activision website (1998)
https://www.mobygames.com/game/hexen-ii/promo/groupId,23130/

Mageslayer cover by Brom:
(not related to Heretic/Hexen series, but very similar in tone and art design)
https://i.imgur.com/U34rMUA.jpeg