Anti-Transformation Powder is magical powder with the ability to undo any transformation spell. Rumplestiltskin plans on using it on himself and Belle so they don't turn into trees when crossing the town line, but he ends up using it on Merida after she turns into a bear.
After the Third Curse
The powder returns Merida back to her true self. ("The Bear and the Bow")
Learning that Rumple has escaped by smashing the chipped cup, Emma forces Merida to chase his true love Belle with her arrows and kill her, so as to make the former Dark One stand up and be brave. Belle has decided to take matters into her own hands in her search for her husband, but soon discovers Rumple hidden in the elevator of the library, and so she helps him hobble back to the shop. Merida soon arrives though, having been commanded by Emma to kill Belle in order to make Rumple fight to save her, thus becoming a hero. The two manage escape, with a bag of magic dust, and Rumple drives them towards the town line, only for Belle to make him stop, for she refuses to leave everyone else behind at the Dark One's mercy. Still realizing her husband to be a coward, she storms off, but is soon found by Merida, who transforms herself into a bear and begins chasing after the brunette. Rumple returns in time to save his beloved, however, using the potion to restore Merida to her true form. The three go back to Emma's house where Excalibur is located, and Rumple pulls the sword from the stone on the condition that Merida's heart is returned and she is assured of the safety of her brothers (who are safe in their mother's arms currently). He warns the Dark One, however, that he is now a hero, and that he will defeat her. He and Belle then leave. ("The Bear and the Bow")