Around the world of Hogwarts Legacy are many fantastic beasts for the player to find and capture. Within the Room of Requirement, you'll be able to keep your captured beasts, taking care of them by feeding and brushing them. You're also able to breed your beasts within the room, and this quest from Deek will help introduce you to this mechanic.
The beast that is focused on during this quest is Thestrals, with the player needing to get two. For those who got the digital deluxe edition, you'll already have a Thestral, and that one counts as one of the two needed. To begin this quest, complete the Plight Of The House-Elf quest and wait for the next quest to appear from Deek.
Beginning The Quest And Getting Thestrals
This quest will begin after you have completed Deek's previous quest, The Plight Of The House-Elf. After this, you will later receive an owl from your professor, letting you know Deek has another quest for you. Deek will request you to find and catch two Thestrals and breed them within the Room of Requirement. You'll first need to go and purchase a pen for the beasts, having you go to Times and Scrolls in Hogsmeade and buy the spellcraft for 1,000 Galleons.
After getting the pen, you'll need to capture two Thestrals. If you got the digital deluxe version of the game, you'll already have one to work with and will need one more. It is also essential to make sure you get one boy and one girl Thestral. The easiest place to capture more Thestrals is in the northern Hogsmeade Valley region. Thestrals will quickly fly away from players trying to catch them, and casting Arresto Momentum on them before trying to capture them is key. For those having trouble capturing the beasts, you can always turn the game's difficulty down to story mode, which allows you to capture the beast instantly without having to stall them for the quick time event mechanics of catching beasts.
Breeding Your Thestrals
Once you have the pen and the Thestrals, return to the Room Of Requirement and bring your beasts to the new Vivarium that opened up earlier in the quest. You'll need to release the beasts within this room and build the pen within the room. After creating the pen, interact with the stand on the right side of the pen and select the Thestrals to breed.
You'll now have to wait 30 minutes of in-game time to continue the quest. After the 30 minutes are up, the quest will update and return to the new baby Thestral. Find the new baby Thestral in your Vivarium, feed it, and brush it. After completing these tasks, return to Deek and complete the quest.