This page offers a guide on where to locate and how to complete Walhdenu Offering Altar, which is a Mystery found in Essexe.
Offer up 50 Iron
This Offering Altar is located north east of Sancta Maria Abbey. As far as offerings altars go, this one is probably one of the easier one since iron is abundant across the land. It is likely you already have 50 iron in your inventory, but on the off chance that you don’t, it can be found easily in mountainous/rocky areas as well as dotted around the rural landscape too. Iron ore are sparkling black rocks and you can mine them by hitting them with any weapon.