To start this quest, you need to rescue Morgan in the "Rocky Road to Portmeyron" quest. Talk to Lev (1), who is atop the castle, and he will take you to the forge. You cannot take your horse with you, because it is too heavy.
Once you are there(2) there is a portal on your right(3). Enter the portal and it will take you to the forge's third level (4). Inside, you will encounter numerous Dark Elves, but keep pushing ahead and you will discover a portal to level 2 (5). There are not many opportunities for exploration here, but there are quite a lot of enemies.
Do not worry about the fact that you cannot cross the door sealed with various symbols, you will receive a separate quest for that, called "Forge of Heroes".
At the second level, (6) you need to keep going down, and then turn to reach another portal (7). Along your way, you are going to fight familiar enemies.
It is a good idea to have some AOE spell on you and a healing spell, because you will get surrounded many times. What is recommended here is the fireball and regeneration.
You need to kill all the elves, because one of them has an earth fragment that will allow you to clear the chapel later on.
Once at level 1(8), go up the map and take the left turn at the fork. You will encounter a mysterious elf there, who you will have a short chat with. After the conversation, go ahead and, thanks to the earth fragment that you have obtained, you will be able to gain access to the chapel.
You need to watch out, however because, as soon as you interact with the altar(10), there will appear a boss that is the earth lord. You will have to fight him but, it is better not to start the fight at all, without spells like Regeneration, Burning Determination, which will stop the boss from turning the part members into stone, and Stoneskin. With these spells, you should not run into major problems during this fight.
After the battle, you will receive a blessing of the Earth Goddess and you will be able to return to Morgan to report.