Champions of the Blood War is an incredibly thorough and rich supplement to Baldur's Gate: Decent Into Avernus adventure. It includes new player options and powers for each class, as well as additional character backgrounds that (in my opinion) add thematic depth to the adventure. For $5, it's a steal in terms of value.
What I'm personally excited to try in my next adventure is the Hellfire Warrior, Fighter—which is reminiscent of the Blood Hunter subclass but that's more thematically focused and devilishly brutal.
I'll add more once I try stuff out and come up with observations, but upon first read, there are fantastic ideas here.