If you're looking to find out how you can solve the "Wake up Art Cullagh" sidequest in Baldur's Gate 3, then you are in the right place, my friend!
If you saved the Druid Grove and recruited Halsin into your camp in Act 1, you can talk to Halsin after entering the Shadow Cursed Lands and start your quest to rid the Lands of their curse.
Once you speak to the Druid in your camp, he will tell you about his friend, the land spirit Thaniel. Halsin believes that Thaniel could heal the land, but he doesn't know where Thaniel is.
Why You Should Wake Up Art Cullagh
After you reach the Shadow Cursed Lands, one of your first destinations will be the Last Light Inn, which is protected from the Shadow Curse. Not all that inhabit this Inn are free from this curse, which you will soon find out. If you enter one of the Inn's side rooms, you will find one of the cursed, Art Cullagh.
When you walk up to him and try to talk with him, he keeps tossing around and muttering a name – Thaniel. Maybe Cullagh can help you find out where Thaniel is. After all, there has to be a reason why he keeps muttering his name, right? But how do you wake Cullagh up?
If you return to camp and tell Halsin what you just witnessed, he will immediately go to the Last Light Inn by himself to check on Cullagh. Once you leave camp and return to Cullagh's bedside, you will find Halsin waiting for you there. Talking to Halsin will reveal that you need to find an item that was of significance to him in order to wake Cullagh from his curse.
Should you not have Halsin in your camp, the procedure to wake Cullagh up remains the same. You need to find an item to wake him up. But what item could we need? Potions and Spells don't seem to work, so what will?
Well, if you talk to Cullagh's caretaker Fist J'ehlar, they will hand you Cullagh's last letter, which reveals that his least order was to investigate the House of Healing. There, you will need to find the item "Battered Lute" to wake Art Cullagh up.
How To Get The Battered Lute
The House of Healing is located southwest of the Last Light Inn, near the Mason's Guild. Make sure to grab yourself a Moonlantern or a Pixie Blessing before beginning your travels, to get through the Shadow Curse.
Once you reach the House of Healing, you will find Malus Thorm in the back of the building. He's the guy you're looking for, as he has the Battered Lute.
Upon entering the operating theater, you will see Malus Thorm in the middle of the room "operating" while the Sisters next to him watch and listen to his lecture.
From here on out, there are two ways to approach the quest. You can either decide to attack Thorm to get the Lute, or you can try talking to him first. Either way, Thorm will have to die in order for you to receive the Lute, but depending on the approach that you take, you won't have to fight him yourself.
Should you decide to immediately attack him, you will enter a boss fight against him and the four Sisters. Just as a quick note, Thorm has 276 hit points and will start multiattacking once his HP reaches 110, so this fight probably won't be a walk in the park.
If you decide to take the second, less battle-heavy approach, I would recommend following these steps:
Before you talk to Malus Thorm, go to the north of the operating theater to find a small, locked room. You can either attack this door to break it, or pick its lock. Once inside, you should look for a book called "Surgery and Physiology: A Sharran's Primer" and read it. This will give you an advantage to avoid having to fight Malus.
After you've read the book, head back to the theater and walk up to Thorm to start a conversation. You'll have to try and convince Thorm to not start dissecting you and your party immediately. If you flat out refuse to be dissected, a boss battle with Thorm will commence. (This is why you need to read the book before you enter the conversation.)
If you successfully roll to avoid being dissected, you can either choose to suggest that the Sisters practice on each other, or to practice on Thorm. By reading the book before this conversation, the DC will be lowered for the "practice on Malus Thorm" option. Should you succeed on all social rolls, Malus Thorm will be immediately killed without you having to fight him. Fail any of these rolls and a battle will begin immediately.
Once Thorm is dead, scavenge his pockets to find the Battered Lute.
After obtaining the Battered Lute, head back to the Last Light Inn. Go to Cullagh and play a few notes for him on the flute to wake him up and fulfill the quest!