Grendelwulf’s Axe & Hammer blog reprinted an article by Gary Gygax from the May 1974 issue of Wargamer’s Digest recounting a D&D adventure with Mordenkainen at 13th level and Bigby at 11th level. They are later joined (after things go wrong) by two fighters (Lords Yrag and Felnorith) and a cleric (Bishop Raunalf). In this adventure, Mordenkainen is turned to stone, Yrag dies, and Bigby is also turned to stone. Old school adventuring is dangerous. The article is a great read, you can find it here: Gygax Legendarium: Swords & Sorcery – In Wargaming(May 1974).