or to party level 0 pc's.They’re quite good! In a nutshell, survivors are simple NPC statblocks that come in 4 varieties; very similar to 1st-level Sidekicks. However, instead of gaining levels in a simplified version of one of the core 4 classes as Sidekicks do, Survivors only gain an extra hit die and Max HP increase (rolled only; no option to take the average is presented), and can pick one of a small handful of new abilities when they level up. They’re very simple abilities, ranging from “gain one additional 1st level spell slot and learn one 1st level spell from the cleric/wizard list” to “you can use your reaction to take a hit for someone within 5 feet of you, and reduce the damage if you’re using a shield” to “when you fail a saving throw you can scream to add a bonus to the roll that can potentially turn it into a success, but the scream can be heard from up to 300 feet away.” They also only go up to 3rd level by default.
The book suggests that Survivors are intended to be used temporarily; either for short one-off adventures, or to allow the players to portray doomed NPCs in nightmare sequences, flashbacks, and the like during longer campaigns. However, there’s no reason you couldn’t run a full campaign with Survivor PCs, if you really want to make the PCs feel powerless.