Baldur's Gate II (5e campaign log)

[Ionathus]

I'm actually introducing my players to Athkatla right now: they haven't met any characters from the game, but the setting and the map and the organizations are all the same! It's been very, very nice to have them say "okay we want to go to the poor part of town" and I can say "why yes I do just happen to have a photographic memory of every detail in The Slums"

Your rules changes on the whole look pretty good. I don't think you need the HP nerf - I'm running a high-level 5e campaign right now and I can threaten the characters' lives just fine even when they have high HP values. Keep in mind that a Fighter in SoA:ToB could end the game at level 40, so the HP nerf was more necessary, but you're only going to level 20 (in 5e). The players are going to need every hit die by the time they're facing multiple liches, dragons, Demogorgon, etc. A single Meteor Swarm is gonna chew through that HP in the blink of an eye.

I like your change for Energy Drain but I think you should simplify it even further: for every level of drain, add a -1 modifier to every roll they make. Lesser Restoration removes 1 level, Greater removes 2-3 or whatever you like best. I think OneD&D UA tried that with exhaustion at one point and I thought it was pretty elegant, and stole it for my 5e game.

Careful with the named NPCs. They're likely going to be precious and interesting to you, but you have no idea whether the players are gonna friggin' hate Yoshimo and Imoen and instead hyperfixate on Ribald Barterman and ask him to come with. My advice is to keep them in the background, make them quest givers and occasional guest team members in a single fight or two, but mostly let the party determine its own dynamics. Don't involve Imoen in the main storyline - give them another reason to care about Spellhold.

Finally, and this is the only big one that I think is going to make or break the game: just friggin' make them all Bhaalspawn. Nothing in the game or the story breaks if you just say "you all grew up as orphans in Candlekeep, raised by Gorion, who was then slain by Sarevok, and you found out you were the children of the dead God of Murder just like Sarevok."₁

Jon Irenicus and Bodhi can steal multiple souls each, why not, you get to make the rules. It makes just as much sense to say "yeah the grafting process doesn't give them the full power, so they needed multiple bhaalspawn for the ritual" and then everybody's got a Stolen Soul^TM. There are so many Bhaalspawn running around in ToB anyway that it makes just as much sense, and then you both A) don't have to jump through any hoops to prevent a few characters from becoming Main Characters and also B) you have a very neat, very simple way to keep the party together and motivate them.

[Edit] Hell, I just had an idea: have Jon Irenicus attack Candlekeep, kill Gorion, and kidnap all of the Bhaalspawn PCs for his experiments! Boom, it cuts you right to the good stuff in BG2, it cleans up the backstory stuff, and it gives players a strong incentive to chase Irenicus to Spellhold.

1. And honestly, if you're not gonna let them play an abridged version of BG1, maybe just shorten it to "raised at Candlekeep as orphans and then the monks told you your birthright once you came of age, but they couldn't keep you safe so sent you on your way" and cut out the Gorion and Sarevok bits, or move the Sarevok bit to elsewhere, if they're not going to actually experience the BG1 story.