Final Fantasy I & II: Dawn of Souls

You'll find the stone slab in the top floor of the Sea Shrine. This room has many treasure boxes and mermaids, but you have to go to the Northwest Corner of that treasure room and walk west to wrap around the room to its east side. Many NES games featured the wrap-around function, which is like going around the world, but that is a long discussion.

Travel to Melmond with your new slab and look for Dr. Unne in the Northern part of the village. Talk to him and he'll happily teach you Leiffenish, the language of Leiffen.

Travel to Leiffen, which is on the Northeastern continent. Land your airship on a small patch of grass east of the desert. This patch of grass will be just northwest of a lake. This will be the only place you can land. Don't forget where you parked, ha ha! Once you land, travel east as far as you can go, and then south until you reach Leiffen. Talk to one of the people on the eastern side of the town. She'll give you the bell to open the Mirage Tower. Next, head north and then east to find the hidden part of Leiffen where you can buy two Level 8 spells for your Black and White Wizards.