Merlin's Beard! Trophy in Hogwarts Legacy

Merlin Trials are 95 short puzzles around the map, each started by placing Mallowsweet on a stone circle. The first comes with the main quest Trials of Merlin in Lower Hogsfield. Grow Mallowsweet yourself: seeds cost 200 Galleons at The Magic Neep in Hogsmeade. Several trials need spells from the assignments, so learn them all first. Once a trial starts, the objects that are part of it shimmer green, and Revelio shows them too. PowerPyx's Merlin Trials guide has a screenshot of where each trial is and how each kind of puzzle is solved.