The Sunstone (also called Stones of Sunlight in the original Dragon Quest release) and the Staff of Rain are two key items related to the main quest. You’ll find one on Tantegel Castle and the other on a cave west of Kol. Together they make the Rainbow Drop, an item needed to access Dragonlord’s Castle and the final boss.
Finding the Sunstone
When you reach Rimuldar for the first time, you can buy Magic Keys from a shop on the outskirts of the city. As soon as you enter the city, go up instead of crossing the bridge, and follow the river until you see the shop. Buy 6 keys from the seller for immediate use and to have later in the game. You can also search the two jars inside the shop to get gold and a Seed of Resilience.
Now that you have Magic Keys, go to the inn at the center of the city and open the locked door. The NPC on the bed will tell you that the Sunstone is located at Tantegel Castle. There’s also a chest in the next room with Wings; you can use them to get back to Tantegel quickly.
When you’re back at Tantegel Castle, there are many doors that you can unlock now with the Magic Keys. Opening the locked door at the northeast will let you walk outside the castle walls. Walk south hugging the wall from the outside and also the lake until you see stairs going down.
Go down the stairs and talk to the old man there who will tell you he’s been waiting for you and will let you access the chest with the Sunstone. Search the drawers in the next room for a Medicinal Herb and Seed of life and leave for the Rain Shrine.
Finding the Staff of Rain
To get the second piece to form the Rainbow Drop, you’ll first need to have found the Lyre of Ire on Galenholm. You can check our other guide on where to find it.
Once you have the Lyre of Ire, you’ll need to head to the Rain Shrine located southwest of Kol. You’ll see a narrow passage with water on both sides. Cross this passage and follow the path north to reach the Shrine.
Give your Lyre of Ire to the priest inside the Shrine and he’ll let you access the chest with the Staff of Rain. With both the Staff of Rain and the Sunstone, you’ll now be able to make the Rainbow Drop! Exit the Shrine and cast Evac or use a Wing to go back to Tantegel.
Making the Rainbow Drop
To create the Rainbow Drop, you need to reach the point of the story in which you already discovered Cantlin and found the Mark of Erdrick. You can check our guide on how to find the Mark of Erdrick.
Once you’re at this point, head to Rimuldar. In the southwestern building one of the NPCs will tell you about a shrine to the south of Rimuldar “where sun and rain meet”. This is the Holy Sanctum and your next destination.
Exit Rimuldar and head south, past the bridge and the small poisonous swamp until you see a shrine surrounded by mountains. Go around the mountains and enter the shrine through the south side.
Once you’re inside the Holy Sanctum with the Mark of Erdrick, the Sunstone and the Staff of Rain, the priest will convert those last two artifacts into the Rainbow Drop.
With the Rainbow Drop you can now access the Dragonlord’s Castle by using it on the small peninsula to the northwest of Rimuldar. In this castle you’ll find the final dungeon, Erdrick’s Sword and the final boss in the game.