Demons and Dreams is a bundle of extra content to play through after beating the Ender Dragon. You will be able to make guide books that give you the recipes for all the items, but the rest is up to you to discover. Most recipes in the guide book do not tell you what they craft, so it's up to you to figure that out on your own.
The new blood moon event provides high risk and high reward. Additionally, once you beat the Ender Dragon, new mobs will begin spawning. The biggest additions in the pack are the new weapons and bosses. Because it's a datapack form, it can run on vanilla servers so long as everyone has the resource pack.
Note that you'll also need the Resource Pack installed on each client: https://www.curseforge.com/minecraft/texture-packs/demons-and-dreams-resources
The Demons and Dreams datapack cannot be updated beyond 1.17.x. Instead, I'll be working on making a Demons and Dreams mod for 1.19 that should be even better. To follow updates, check out the discord: https://discord.gg/gbeVkSaauR