Complex sentence: Sales of "D&D" reached in 1985, but Gygax, seeing his future at TSR as untenable, resigned all positions with TSR, Inc. in October 1986, and settled his disputes with TSR in December 1986.
Split into simpler sentences: Sales of "D&D" reached in 1985, but Gygax, seeing his future at TSR as untenable, left the company in October 1986. He resigned all positions with TSR, Inc., and he settled his disputes with TSR in December 1986.