Synopsis

In order to avoid spoiling anything inadvertently, I will do so immediately. This also aids in discussion as I find that most Dungeons & Dragons books either focus on a specific character or group of characters over all else, or specifically on a story that needs to be told.

So Tristan Kendrick is the son of a local king, and his adoptive sister is an orphan. And they have a halfling thief friend that tries to cheat them out of as much gold as possible (what else would a halfling thief do), and a newcomer to the region that's more or less an assassin/spy hybrid that also knows how to train dogs real well. Which is convenient, since there's a new dog. And a bard, because everyone needs a bard.

The eponymous Darkwalker is an unnatural corruptive beast/force that wants to bring down all of humanity (probably), but especially the Moonshaes because someone there was the one who sealed him away in the first place. He sends a werewolf to lead a giant wolf pack, stirs up the Firbolgs - sort of naturey giants which have a horrible reputation as most giants due despite Firbolg's more recent interpretations - and personally leads a group of northmen himself.

Throughout the book, Robyn - Tristan's adoptive sister and love interest - discovers that she's the daughter of the Great Druid of the land (presumably, it's never spelled out that clearly how powerful her mother is other than powerful), which imparts a sorcerer-like control over nature (despite sorcerers not being canon yet, and druid-sorcerers never being a thing as far as I'm aware - who knows, I wouldn't put it past 5e archetypes), and is the key to getting rid of the Beast.

Tristan and his companions - including Robyn from above - go on a quaint little quest, find the artifact that everyone cares about, and end up leading entire armies against the combined forces that the corrupted Beast has brought to bear. Except for the wolf herd, which the dog just took leadership of essentially off-screen (i.e. largely through itallic end-chapter text). They break the attack and the band of five (comprised of 2 PCs and 3 NPCs) chase down the big bad and defeat him.

Except... one of the corrupted druids takes his heart. Because you can't have a fantasy book that's less than a trilogy, even back in these days.