Once you hit the Shadowlands in Baldur's Gate 3, you'll discover a man who is catatonic inside the Last Light Inn. While this may seem like a fairly small side quest, it's actually key to completing the overall goal of Act 2 of Baldur's Gate 3.
How do you wake up Art Cullagh?
In order to wake up Art Cullagh, you need to head to the House of Healing in the Shadowlands. This is in the southwest with regards to the Last Light Inn itself, and you'll discover somebody inside that building called Malus Thorm. Thorm is surrounded by corrupt individuals who are acting like they're members of a sisterhood. You need to talk your way into seeing Thorm, and you'll be given the option to do a few things here.
You can either just attack Malus, which means fighting both him and his entire squad of corrupted Nurses, or you can persuade him to undertake his own surgery method. If you do this, you'll discover that his surgery isn't actually as good as he says, and he'll be immediately killed. You can now loot his corpse to take the instrument that he stole from Art Cullagh, and use it to wake him up.
Once you wake up Cullagh, you'll discover that he's key to finding a way to lift the Shadow Curse that has afflicted the entire area that Act 2 takes place in. Through both him and the Druid Halsin, you'll be able to fix this land for everybody.