Of all the old Fighting Fantasy Gamebooks set in the Old World this is by far my favourite - and incidentally the first of such books I came to own and read.
The player is a Demon Slayer whose parents have been kidnapped by the Demon Prince Myurr. The plot revolves around the player's mission to rescue them, travelling through various Demon-blighted regions and meeting up with old friends and allies in their homeland.
In addition to the regular attributes, Dead of Night features evil which start with an Initial score of 0. Throughout the book the player may encounter people or creatures, or do things that will increase this score. As a result the player is asked to Test Your Evil at certain points in a manner similar to Testing Your Luck. Furthermore, the player also has a choice of three "Talents" from the following list: Banish Undead, Dark Veil, Heal, Holy Circle, Meditation, Sense Demon, and Speak Demon.
One of the darker-toned originals, it's a shame that the revival of the Fighting Fantasy Gamebooks (and some fans) seem to overlook the Old World as an incredibly rich and diverse setting. This is the book (in my humble opinion) that truly brought the continent to life!
I reserve judgement as to whether Stormslayer continues the dark legacy - or indeed future gamebooks, such as Night of the Necromancer.