You'll have to track down Bourgeon skin in Clair Obscur: Expedition 33 if you want to help the small Bourgeon grow up to be big and strong. While exploring the continent, you may have happened upon The Small Bourgeon region, which contains one of these little Nevrons who asks for your help to get taller like his older brother.
To do so, however, he needs to engage in some light cannibalism and eat the skin of another Bourgeon, which will help him get tall. Where do you find this skin? Well, that's what I'll cover down below, as well as explaining how to best beat the Bourgeon boss and what you'll get as a reward for all this trouble.
Where to find Bourgeon skin
You can find the Bourgeon skin in the Flying Waters area; specifically, you get it by defeating the optional Bourgeon boss who's close to the Expedition 48 – Lumerian Streets rest point. Since you go here during the main story, simply head back the way you came through the alley and turn left through a small ravine to find a rocky arena with a Bourgeon.
If you've progressed to the point where you unlock Esquie, you could also fight a different Bourgeon on the path to the right of the Stone Wave Cliffs entrance, and there are multiple Bourgeon in later sections of the game.
How to beat the Bourgeon
The Bourgeon mini-boss can be a little tricky since it swallows members of your party, removing them from combat. If you use Gustave's Overcharge and other abilities that "break" enemies, the Bourgeon will spit your party member back out. Breaking a Bourgeon will also reset their timer for eating one of your party.
I personally recommend using lightning (Bourgeon is weak to it) or burn to stack up DoT with Lune, with Maelle's Degagement ability to inflict fire weakness and chip down its health even faster. If you're looking for an early lightning weapon, you can get the Deminerim for Lune by helping the Demineur. You could also use Maelle's Percee ability while in Virtuoso stance, which is powerful even quite early in the game provided you're levelling her attributes right.
Once defeated, bring the Bourgeon skin back to the small Bourgeon and after enjoying its snack, it tells you to return later. You can just exit and re-enter The Small Bourgeon to find the monster is now fully grown. He'll thank you and spit out a Colour of Lumina as a reward.
Should you attack the Bourgeon in Clair Obscur: Expedition 33?
As with other Nevrons you help in quests, such as the Jar or the Demineur, you can also fight and kill the small Bourgeon afterwards if you choose, though he'll be rather sad about it. This Bourgeon has the same attacks as the previous you fought, and if you defeat him you'll get five Colour of Lumina and one Polished Chroma Catalyst. As far as I can tell, as with killing the other quest Nevrons, there's no consequence for defeating him other than any guilt you might have.
