Fafhrd and the Gray Mouser influenced DnD, D&D

Leiber is in Appendix N and the debt is unusually concrete for that list:

• The thief class is the Gray Mouser. A lightly armoured specialist who solves by stealth rather than by combat did not exist in the 1974 rules and arrives in Greyhawk the following year
• The pairing of a big northern barbarian with a small city rogue is the party's original shape, and Leiber invented it
• Lankhmar is the model for the fantasy city as a dungeon: guilds, patrons, jobs, and a thieves' guild that must be negotiated with

TSR licensed Lankhmar outright and published Leiber's world as an official setting from 1976, which is the acknowledgement in contractual form. Leiber also coined the term sword and sorcery, the genre label the game was built to sit inside.