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Beginning your adventure in Baldur's Gate 3, you'll have the option to recruit multiple companions to your party under the coordinated effort to relieve your minds of the same mysterious parasite. Of course, every good adventuring party has its healer, and your first opportunity to recruit one comes in the introduction of Shadowheart, the Trickery Domain Cleric.
However, getting Shadowheart over on your side isn't so straightforward. She's the first companion you could potentially miss, and failing to recruit her in the opening scene means missing out on a healer for the beginning of your game - a potentially catastrophic handicap so early. You'll want to make sure you free her as soon as the opportunity presents itself.
Freeing Shadowheart
You encounter Shadowheart in a Mindflayer pod shortly after the first combat encounter with Lae'zel at your side. She'll be trapped inside, and trying to pry open the pod with your bare hands or bashing the control panel isn't going to work here.
You can speak with Shadowheart briefly in the pod to reassure her you'll find a way to let her out. Once you're ready, continue past the pod and the control panel and head toward the eastern exit (the door just right of the control panel).
In the hallway will be a patrolling Intellect Devourer, but you don't need to fight it. Unless you're vying for the little extra experience, ignore it and run past into the next room, where you'll find a second mind flayer pod.
You can choose to interact with the control panel or ignore it. Your true goal is up the stairs and on the corpse at the top. Loot the body to discover an Elaborate Key. Take it and return to the main room.
Interacting With The Unnamed Woman's Pod (Spoiler)
Interacting with the control panel in the eastern room will turn the woman in the pod into a mind flayer. This is meant to put the seed of doubt into the player's mind about using the control panel to free Shadowheart. You cannot save the unnamed woman inside this pod.
Returning to the room with Shadowheart, turn immediately left as you enter to notice an "Elaborate Reliquary" (the chest) in the corner. Your key will unlock this chest and reveal an Eldritch Rune; Loot it.
Taking the Eldritch Rune back to the control panel beside Shadowheart, you can now interact with it and select:
Insert the rune into the socket.
Place your hand on the console, and you'll be informed of something "squirming inside your head." This is your first potential use of the parasite placed in your head, and you'll note that any such interactions like this from now on may come with an "Illithid" (Wisdom) check. The DC here is zero, so you cannot fail, but you may experience unique consequences for using this power more in the future.
Use your Illithid power to "Will the pod to open," which will set Shadowheart free.
Once Shadowheart is free, she'll join your party whether you want her or not - at least for now. With the upcoming boss battle, she'll be a notable asset to you.
Haul Shadowheart with you to the south exit, where you'll soon need to fight Commander Zhalk and his forces of Avernus. Good luck, and keep your Cleric close.