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After taking the Mountain Pass to the Githyanki Creche in Baldur's Gate 3, you'll encounter a curious NPC: Lady Esther wants you to steal a Githyanki egg from nearby Creche Y'llek under the Rosymorn Monastery so she can give it to the Society of Brilliance (you know, those nice men you found at the mushroom village in the Underdark). Luckily, it's easy to do this without ticking off Lae'zel or the inhabitants of Creche Y'llek, as it turns out they won't really miss the egg anyway. I've organized the quest by stages so you can home in on any pain points.
Finding Lady Esther
You'll find Lady Esther in a little switchback cul-de-sac on the mountain path down to Rosymorn Monastery. She's also a merchant with great wares: The Graceful Cloth is the best Rogue or Shadow Monk armor I've found thus far, granting +2 Dexterity and permanent advantage on Dex checks like Stealth or Sleight of Hand while you're wearing it. Lady Esther closes shop on completion of her quest, so grab any goodies ASAP.
I demurred on taking the quest at first to avoid a Lae'zel disapproval hit, but this doesn't lock you out of it, and the whole thing winds up much less tawdry than Esther makes it sound—it's possible to complete this quest with Lae'zel in your party the entire time without running into any approval hits or confrontations.
Getting into the Githyanki Creche
There's a fairly linear path from Lady Esther down to Rosymorn Monastery. At the first fork in the road, go east (west leads to a tough fight and the beginning of Act 2), and at the second fork you can either pass a Strength check to unstick the gondola, or take the lower path to the west, but watch out for repulsion traps.
The front gate to Rosymorn is locked, and requires exploring the upper floors to go around. To the west of the entrance are some broken windows you can jump through, and you'll encounter some not-too-challenging fights with Kobolds and some weird little cat monsters on this route. You can also turn to the south and find a lower path to the monestary's east wing that bypasses combat altogether.
You can thoroughly explore the upper floors of the monastery to find the start of the related Blood of Lathander quest, which'll net you a sweet little mace at the end of the Rosymorn/Creche Y'llek arc.
Creche Y'llek: steal the egg or persuade Varsh Ko'kuu
The hatching chamber is located in the western portion of Creche Y'llek, just north of the infirmary you have to visit for both the Find a Cure and The Githyanki Warrior quests. While you can potentially get around the acid pool with jumping/teleporting and steal the egg the old fashioned way, I recommend talking to Varsh Ko'kuu instead.
Jump or teleport up to his perch in the center of the birthing chamber, overlooking the egg, and talk to him. Asking why the chamber is so empty of eggs prompts an automatic Wisdom (Insight) check. If you fail the check, you won't have the opportunity to persuade Ko'kuu, but I was able to pretty consistently pass without proficiency in Insight or a Wisdom bonus. Consider save scumming here to ensure you get the option (c'mon, we all do it).
You then have to pass two Persuasion checks, DC 14 and DC 16, to ask about the egg and get Ko'kuu to give it to you. I don't know if the final dialogue choice affects the outcome, but I told Ko'kuu I was taking the egg to a kinder, gentler Creche, and maybe there's some truth to that. It's 1492 DR, people! Sometimes a Creche is a genius Hobgoblin, a repentant mind flayer, and a kooky lady who sells cool clothes. Taking the dialogue route ensures the Creche (and Lae'zel) remain friendly to you.
Optional: use an owlbear egg instead
It is possible to give Esther an Owlbear egg and pass it off as a Githyanki one, but I don't know what the long term consequences of this action are—acquiring the Owlbear egg in the first place seems to be mutually exclusive with rescuing the Owlbear cub though, so why would you even consider it, you monster?
If you have the Githyanki egg, tell Esther about it, then refuse to give it to her, she becomes hostile. There may be longer-term consequences to this choice later in the game, but it is currently unclear if there are any future benefits to having the egg in your inventory.
