Go to the Pokémon Center and heal and save. Walk behind Eagun's house, and down the hill. You will come to a chest containing the Exp. Share. This is a very helpful item. Equip it to a Pokémon to have it gain levels faster.
On the most northeastern ledge in Agate Village, there is an old man who hands out Berries that his Taillow picks—akin to a Berry Master in other regions. The type of Berry he gives out is randomly selected from the first fifteen Berries obtainable in the Hoenn region.
Orre's only Day Care is located in northeast Agate Village. The Day Care has room for only one Pokémon, making breeding impossible. The Day Care is also home to Orre's only Name Rater. The Day Care can assist in purifying Shadow Pokémon as well (a feature that is removed from Pokémon XD).
Go to the giant tree in the middle of town. The people inside of here are Rui's grandparents, Eagun and Beluh. Talk to Eagun and someone will come in saying there's trouble at the Relic Stone! Eagun runs out to help, despite his old age.
If you go down the path south of the cave leading to the Relic Stone, you can find a Quick Claw.
After the battle, you will go back to Eagun's house to discuss the Relic Stone with him. After the conversation, watch the news report. Eagun sends you to see a man named Senilor. Go southeast of the Poké Mart. Senilor tells you about the Relic Stone. Go back to Eagun's house. Beluh gives you the Small Tablet. Now, you can purify your Shadow Pokémon at the Relic Stone. As you exit Eagun’s house, you get an email on your P★DA from Duking.
Subject EMERGENCY!!! SenderDuking of Pyrite Town Wes, we've got big trouble! I just got word from our people that Mt. Battle's under attack by a mysterious group of thugs. Please, can you go help? We think it's the same group as the one Miror B. belonged to. We're counting on you!
Go back to the Relic Stone and pick up Ein File C, a report on Celebi. Press 🅐 on the Relic Stone and select a Shadow Pokémon in your party that is eligible for purification. It is possible here that the EXP your Pokémon have accumulated while being a Shadow Pokémon will cause it to evolve.
There is a glitch you can exploit because the cliffs in Agate Village directly to the left and right of the hill at the entrance, as well as the top edge of the Relic Stone, are buggy. While walking down/up on these small cliffs or edges, there are specific points where the player will hit the edge, continue to walk, and cause the screen to start to shake. This can be used to automatically purify all Shadow Pokémon in the player's party or the Day Care.