The Merlin Trials are scattered throughout Hogwarts Legacy, waiting to be discovered and solved. Two are located near Falbarton Castle, to the northeast of Hogsmeade. You can find these fairly early on during your playthrough, and you might them while helping Natty get into Falbarton Castle.
How to Finish all Merlin Trials at Falbarton Castle in Hogwarts Legacy
At Falbarton Castle, there are two Merlin trials you may complete. Both are west of the castle, with the one being closer than the other. This is the first Merlin trial.
How to Find Falbarton Castle in Hogwarts Legacy
Falbarton Castle's first Merlin Trial is just west of the castle. You may reach this location by broomstick or walking there yourself.
Some butterflies will appear in front of you when you activate the pedestal, and they are glowing. This puzzle is similar to the moths you've encountered in Hogwarts Legacy. You need to invoke the Lumos spell and drag them to one of the three pedestals in this area.
One of these blocks is visible immediately behind the starting location of the Merlin Trial.
Related:
Another will be near the start of the Merlin Trial, on the top of a small cliff. There are glowing butterflies below the block that you can place.
The final batch of butterflies is a little more difficult to find. They will be found in a cave southeast of the starting location for the Merlin Trial. Go into the cave, and bring the butterflies out.
Once you have those butterflies, head to the west, where you'll find the final block next to a rock wall. This will be the final block, and you'll have completed the first Merlin Trial.
The second one will be relatively small, so you can go to the west and find it on your map. When you arrive, activate it, and a ball will appear on the top of the hill.
Make your way to the ball, and you solve this Merlin Trial by putting Depulso on the ball, causing it to swing backward.
This will take you down the hill, and when it reaches the bottom, use Accio to hold it in place and ensure that it settles in the center.