Foal of the Dead is an optional quest in Hogwarts Legacy. Deek believes that you might be interested in learning how to breed Beasts. He would like for you to go see him in the Room of Requirement, so he can show you how.
To begin Foal of the Dead, you will need to speak to Deek. Use the Room of RequirementFloo Flame, and he will be to your right. Talking with him (with /) will begin The Foal of the Dead, where Deek would like for you to do two things in order to begin breeding Beasts: he wants you to capture a male and female Thestral, and purchase a Conjuration that will allow them to breed.
The next two steps can be done in either order.
Purchase a Breeding Pen Spellcraft
You will need to purchase the Breeding Pen Conjuration from the Tomes and Scrolls store. To get there, just use the South Hogsmeade Floo Flame, and head to the south part of the area — the store will be on your right. Once inside, look for the Breeding Pen Spellcraft to purchase — the game will have two lights moving around its icon in case you have trouble finding it. The Conjuration will cost 1000.
Capture Two Thestrals
If you purchased the Deluxe Edition of Hogwarts Legacy, the Thestral you got from it will count towards this quest, meaning you will only need to capture one of the opposite gender.
You will now need to find the Thestral den, which will be on the northern end of the world map. If you happened to have unlocked the East North Ford Floo Flame, you can just go there and briefly fly west. Otherwise, you can go to Hogsmeade (if you’re not there already), leave from the northern end, then get on your broom/mount and fly north until you reach it.
The Thestrals are surprisingly timid, and they will easily outrun you, so you will want to use Disillusionment to have an easier time approaching them. You will need to catch both a male and a female, so pay attention to the gender symbol that shows up next to their name while targeting them.
Once you’re close enough to catch one, cast Levioso on it, then capture it with your Nab-Sack. You will have to attempt to capture it five times before it’s successful (you will see the dots under him track your attempts). Because of how fast the Thestral can run, you will want to recast Levioso as soon as it wears off, or risk losing it.
Once you’ve successfully captured one, repeat the same process for a Thestral of the opposite gender.
After finding a good spot for your pen, cast the Conjuration Spell, find the Breeding Pen in the Beast Items menu, and create the pen on your chosen spot — it will cost 15 Moonstones to create it. Once created, interact with it and choose a Thestral to breed.
Getting a Baby Thestral
Once you get the breeding process started in the pen, you will need to wait 30 in-game minutes for the offspring to be born. You can kill time however you wish — maybe complete another side quest, or do a main quest while you wait.
Once the half hour is up, you will get a quest notification. Go back into the Vivarium, and you will see your offspring leave the pen. Use the Beast Petting Brush and Beast Feed on it, and you will be ready to report back to Deek.
Return to Deek
Return to Deek, who will be in the same place as when you started the quest. Talk to him to give him the good news about the Thestrals.
After speaking with him, you will have completed Foal of the Dead, and will now be able to breed Beasts whenever you want!