Phoenix on the Sword influenced DnD, D&D

Gygax put Robert E. Howard in Appendix N, the reading list printed in the 1979 Dungeon Master's Guide, and named him among the authors of "immediate influence". The Phoenix on the Sword is the first Conan story, published in Weird Tales in December 1932. What Howard supplies:

• The fighting man as a viable protagonist without magic, which is the fighter class and the reason D&D is not only a game about wizards. Gygax has said the fighter was built to be Conan
• The adventuring premise itself: a wandering outsider entering ruins and cities for treasure, with no kingdom to defend and no destiny to fulfil
• A world of fallen empires with usable ruins in them, rather than a stable setting
• Sorcery as something dangerous and other, practised by people the heroes fight rather than by the heroes

Gygax also named the thief class as Howard's and Leiber's, and the Hyborian Age as the model for a campaign world assembled from historical analogues.