Mallowsweet

Players can buy seeds in Hogsmeade and grow their own mallowsweet plants.

Since the room becomes available after completing a good chunk of the main quests, it is possible to plant the Mallowsweet in the Herbology Classroom’s Pot temporarily and wait for the Room of Requirement quest to trigger.

After this, you’ll be able to find all Merlin Trials, but you’ll also need to make sure you have Mallowsweet Leaves on hand.