Nericon's Garden is a locked-off area north of Stormfist Castle in “Divine Divinity.” If you talk to Blake at the General Store in Rivertown, he'll tell you that he has the key; however, he says he won't sell it to you because your reputation is too low. You must earn an invitation to Stormfist Castle in order to purchase the key and open the garden so that Nemris' worshipers can enter.
Earn an invitation to Stormfist Castle. There are several ways to do this but the easiest is to take General Alix's letter to Captain Mitox, complete his quests (ravaging the orc supply train and poisoning the orc water supply) and return to General Alix.
Ask Blake about his “rare articles at reasonable prices.” He'll sell you the key to Nericon's Garden for 100 gold pieces.
Find the three worshipers of Nemris on the road from Stormfist Castle to Nericon's Garden. Speak with Otto and tell him that you're interested in their religion and receive a book of Nemris' teachings.
Enter Nericon's Garden and kill the frogs in the middle of the garden. This will open a portal and animate the statue of Nemris.
Talk to Nemris and answer his questions about the holy book and his philosophy of life. The answers are: Experience; Truth and Untruth; and Good, Neutrality and Evil. The worshipers will enter to worship Nemris and you'll earn 6,000 experience points and one level of Restoration.
Go through the portal to find a treasure room. Move the packages to reveal a lever to unlock the door. Inside you'll find chests of gold and a frog statuette.