Brimstone (1985), a real-time "electronic novel" by James Paul. Programmed by Synapse Software and published by Brøderbund.
As described on the book:
It is the Eve of All Hollows. Gawain, Knight of the Round Table, goes forth, summoned by Destiny. Across a moonlit plain, a shimmering xastle seems to beckon. On its parapets, a breath of breeze stirs a banner. The knight nears the massive door, knowing nothing of what awaits him behind those walls. Soon Gawain will discover what is known to men only in legends - or nightmares. And so will you. As the underdemon, bat-monkey-man, reaches out his gnarled and furry hand to yours, you catch a glimpse of his wild eyes. They are dancing red with the flickering fire of BRIMSTONE.