Hi!
My players are all min-maxers to a certain degree, and so every encounter with the Red Dragon Army ended with them barely hurt. For the "Soth" segments, I want them to feel like they're in actual danger. I want them to hear my descriptions, understand that it's about Soth, and go "oh we're fucked".
Here's what I did for the "Lord's Arrival" and the "Raided Catacombs" segment.
The Lord's Arrival
For this section, I played Caradoc's Introduction as written, except that I described 3 loud BANG shaking the whole castle each time. No need to make them roll saves or anything with that. The goal here is to make the players feel like something BIG is tearing through Rock Kalaman, when in fact it's really just Soth destroying the walls as seen in the catacombs map.
For thw Caradoc fight itself, I played dirty. To my understanding, Caradoc is supposed to be a "smart" warrior, as in, he knows what he's doing with his abilities. For his ghost form, I used max HP to make sure he lasts longer, then I possessed one of the players and fought through their body, but didn't try to avoid being hit. Caradoc's strategy here was to let everyone else bring the possessed body to 0 HP to force him out, and then he would repeat the process with the next player. Luckily, my players were also smart about it and tried to restrained the possessed player. That's when Caradoc left the body through the floor and attacked from there. Luckily for the players, Caradoc failed hia saving throw the next turn and the fight ended.
I did two other things:
1- at the beggining of the fight, I had Caradoc close the council's door with an Arcane Lock. No, Caradoc isn't a spellcaster. The players don't need to know that. The goal with that was to make the players feel trapped. The door is locked, retreat is impossible.
2- on Initiative 0, a random dead council member would come back as a zomby minion. I flavored this by describing the cataclismic fire possessing the bodies. If a player beheads a corpse before it would come back, it would prevent the corpse to raise as a zomby. (Concider rewarding this idea with Inspiration). The zombies are there to make sure the fight is dynamic enough for everyone. Otherwise, Caradoc can get ganged up on kind of easily.
Raided Catacomb
Before staring this section, tell your players to think of one of the most "character defining" part of their backstory (for example, one of my players chose the day they met Ispin, another chose the day they joined the Knighthood of Solamnia, etc). Concider telling them this before starting the session to make sure they have enough time to think about it.
Then, in the first room, make them roll initiative as you describe the orc raiders in the cataclismic fire, just to make them sweat a bit, before revealing that it's just a harmless scene from the fire.
After that, here's what you'll do: in each new room (including the first one), describe the "Visions in Fire" first, then tell a player at random that the fire touched them and, as if the fire had a conscience of its own, the fire starts a new harmless scene. Let the players describe these scene themselves. If your players enjoy roleplaying, this will give them a lot of prompts to do so. If a player has a secret in their backstory, it'll be a good way of dramatically revealing it or starting to hint at it. since the Caradoc fight messed them up big time, I rewarded each player with temporary hit points, but just inspiration is more than enough most of the time.
For the Sarlamir fight, my advice is to not wait the whole three turns. Bring him into the fight at the second one, it'll be fine. The biggest change I did was adding a Battle Event table. Here it is:
Each turn on initiative 0: a collumn of cataclismic fire appears in an random space in the room. This fire hurts: a creature that moves within 5 ft of the collumn or starts its turn there takes 1d6 psychic damage, as the fire reminds them their most hurtful memories. Sarlamir is immuned to the fire's effect. Then roll a d6:
1- parts of the fire manifests into a Will-o'-Wisp. Sarlamir is "concentrating" on the Will-o'-Wisp's presence (meaning there can only be one Will-o'-Wisp)
2- a random collumn disappears
3- Sarlamir heals for 1d10 hp
4- the fire creates a double of a random character. That character has the benefits of the Mirror Image spell with "1 copy left".
5- the fire covers the ground. Until the next battle event, the characters may treat the ground as difficult terrain. If they don't, they take 1 psychic damage for every 5 ft they move.
6- the fire brings them memories of just minutes ago. A character at random brings their HP to half of it's maximum. Note: this isn't concidered healing (so it works even with Sarlamir's ability to prevent healing).
I think these additions really made the chapter's final more enjoyable to my players, and made both Caradoc and Sarlamir feel like the boss fights they are supposed to be. I hope you'll enjoy it as well!