First: I enjoyed the game a lot. I will only focus on the negative things here to save everyone some time.
Warning: Will obviously contain some spoilers

Flaws/Bugs:
- General problems with Line of sight/ line of effect. A few examples:
PCs were spotted (while sneaking) through a wall at the blighted village ("intruder!" dialogue)
Arrows were shot at PCs through wooden doors (opened) and stone walls in the Shattered sanctum (the same path was sometimes blocked for attacks of the PCs although I can't be 100% certain this part is true, for the NPCs may have moved 1m after taking the action)
on the same note: Hellish Rebuke failed (probably because the path was blocked) after the PC was shot by an archer (also in the Shattered sanctum)

- Wyll complained "you killed Spike without me" even though he was with the party and I think even the one to deal the final blow.

- Auntie Ethiels dialogue bug: she teleported away at the start of the last line (and thus did not finish it, I had to look in the journal to find out what she said)

Suggestions:
- Let a Character delay the initiative (or hold an action).

- Dial down surface effects: It's really cool to ignite something with a fire bolt or to douse flames with a ray of frost. But having a ray of frost freeze the ground for several rounds and knocking people down as a 1st lvl grease spell in addition to the dmg seems a bit strong for a cantrip. Also, a fire arrow setting fire to 3m2 of stone feels weird (unless that's supposed to happen by magic and I missed that^^). On that note, consider to set the effect to happen at the end of the turn (when possible). Currently I think it deals more dmg if a character walks through a flaming surface than spending the whole turn on it.

- characters can't get up from prone (the same turn they're knocked prone) even if they have more than half of their movement left and/or an action to dash. (I don't know if this is intended but feels a bit weird)

- It feels to easy to attack and hide in the same turn (when in shadows, I think there is not even a stealth check). Not sure if that's 5e rules, but IF this is kept, the AI should react to the last known position of the PC (in my experience this has not happened so far).

- Make it possible to target party members for spells via the portrait.

- AI often prioritizes hitting a downed character over their own survival. I would suggest changing that for non-fiend/non-Mind Flayer NPCs (as in: "they are down and no current threat. The guy over there throws spells at me, I should deal with him first")

- Make it possible to enter a conversation with the PC. Currently, when in SP, the warrior might trigger a dialogue, but the "face of the party" can't take over to convince the opponent of something. It would be nice, if a Charismatic character that focuses on talking her way out of situations would not be required to take point in the formation. (might already be intended with the "switch character" option, but I was unable to switch to my main character and intervene in the dialogue)

- Resting system: So far, I was only able to take a short rest once a day. Idk if that's intended. Either way, I was able to rest (both short and long) while the party was hidden and surrounded by enemies (goblins in the Shattered sanctum that were currently hostile and 20-30 meters away). Kind of an immersion breaker.

- Please insert more options once an enemy is knocked out. I like that finally a good PC is no longer forced to kill every bandit. But it would be great if we could interrogate them and/or take them prisoner and so forth. Currently it's a bit counter-intuitive, that one gets more information from a dead enemy (speak to dead), than from a knocked-out, but alive enemy. In my play through I spared the siblings of the dying dwarf after they attacked me (by knocking them out). later, they attacked me on sight in the goblin camp. Was a bit disappointed by that. I would have expected them to at least note, that I went out of my way to not kill them, even if they still wanted me dead^^
on the same note: An option to "toggle non-lethal attacks" or something similar would be great in order to avoid accidentally killing an opponent with a reaction and for comfort when going for a non-lethal play style.

Thank you for creating a very promising game and for taking the time to read through so much feedback!
cheers!