From publisher blurb:
Travel to the Furnal Planes, a set of three dangerous fiendish planes of existence. Explore the maddening plane of Deliria, the breaker of minds. Survive the corrupting plane of The Black Rift, the breaker of bodies. Journey into the fiery Cinderdeep, the breaker of spirits. Encounter blood rains, petrifying meadows, mirror mazes, and crawling walls as well as the horrid new monsters that inhabit them.
You'll find the following within this compendium's 38 pages:
3 new fiendish planes of existence and their dangerous settings
1 new demiplane and the titular Furnal Bridge that spans from it
10 infernal quests into the Furnal Bridge
25 fiendish items, including a sentient soul sword and an animated chain
11 hellish monsters, including 3 deadly archfiends and a new type of fiend called an agon.