He’s finally figured out a way to improve the Royal Watering Can for good and it involves Purified Night Shards. He gives you the list of Ingredients he needs.
The best way to know if the enchantment worked is to test your new and improved Royal Watering Can.
Merlin will be happy to learn that the experiment was a success.
The interface to give Merlin the ingredients for the enchantment may stay up after clicking Give while the dialogue continues. Clicking either Cancel or Give will advance the dialogue, but the interface will remain after the dialogue is finished. Force-closing the game fixes this.