I assume you mean FF2 on the Snes, also sometimes called FF4.
GETTING EXCALIBUR (from GameFAQs)
You can do this side quest at any moment after you get the Rat Tail in the
Land of Summons and the Lunar Whale airship.
To start the side quest take Enterprise and hook the hovercraft, then move to
Mythril Village, there land the hovercraft and enter it, move southeast through
the rocky area, then enter the cave, the Adamant Grotto, inside there's a a man
that collects animals tails, so give him the Rat Tail and he'll trade it for the
rare Adamant Ore, now leave the grotto and move to the underworld, there head
to Kokkol's Smithy, that's located on the southeastern part of the underworld,
there there's a smith that can't work anymore 'cause the Adamant become rare and
he can't find it, so give the Adamant to him and he'll get the Legend Sword too,
to make an upgrade. The smith says to give him some time to make the sword, so
before leaving search for a SomaDrop somewhere here. Move back to the upper
world and then head to the moon. Come back to Kokkol's Smithy and get the supper
powerful sword Excalibur.
I assume you mean FF2 on the Snes, also sometimes called FF4.
GETTING EXCALIBUR (from GameFAQs)
You can do this side quest at any moment after you get the Rat Tail in the
Land of Summons and the Lunar Whale airship.
To start the side quest take Enterprise and hook the hovercraft, then move to
Mythril Village, there land the hovercraft and enter it, move southeast through
the rocky area, then enter the cave, the Adamant Grotto, inside there's a a man
that collects animals tails, so give him the Rat Tail and he'll trade it for the
rare Adamant Ore, now leave the grotto and move to the underworld, there head
to Kokkol's Smithy, that's located on the southeastern part of the underworld,
there there's a smith that can't work anymore 'cause the Adamant become rare and
he can't find it, so give the Adamant to him and he'll get the Legend Sword too,
to make an upgrade. The smith says to give him some time to make the sword, so
before leaving search for a SomaDrop somewhere here. Move back to the upper
world and then head to the moon. Come back to Kokkol's Smithy and get the supper
powerful sword Excalibur.