Instructions
Download and install the latest Oblivion patch if you don't have it. In the original, unpatched version of the game, gates in Ayleid ruins sometimes remain closed when you activate the associated pressure plate. When this bug affects your game, you cannot reach and collect the stone.
Play the game, following the red point on your compass to Miscarcand, which is between Skingrad and Kvatch.
Enter the ruins. Follow the red point on your compass to find successive levels' entrances. On the way, you will encounter several goblins and zombies. You will also, more than once, come upon a sealed stone gate.
Stand on a nearby panel that looks different from the surrounding floor. The gate will open.
Continue following the red marker on your compass. You will eventually reach the Great Welkynd Stone, which sits in the center of a room.
Take the stone. A number of new opponents will appear, including the King of Miscarcand. Kill them all. If you escape without killing them, they will continue to hunt you from afar, preventing you from ever sleeping or fast-travelling.