Once the disk is installed and a previous game is loaded (or a new one begun), an earthquake shakes Britannia, signalling the rising of the Isle of Fire from the oceans. The Isle was last seen in Ultima III, serving as the stronghold of Exodus. And while Exodus was destroyed, some of his essence remains in the Dark Core, which must be destroyed to rid Britannia of his evil once and for all. To accomplish this, several tests of virtue must be passed, originally designed by Lord British as part of the Quest of the Avatar.
The rewards of completing the add-on's quests are plentiful and can make the main game easier: the Avatar's stats will be raised to their maximum and a new, powerful weapon will be made available, able to vanquish even the most dangerous foes with a single strike.
Warner Books published a novel sharing the name of this game, nominally set between Ultimas4 and 5, under their "Questar Fantasy" imprint. It was printed in January of 1991, written by renowned fantasy author Lynn Abbey. Its ISBN is 0445210656 and it is the first in a series of Ultima novels, followed by another one by Lynn Abbey a year later, The Temper of Wisdom, and a Technocrat trilogy by a different author and under a different publisher.
The Forge of Virtue's back page blurb reads:
BRITANNIA... is the wondrous land Lord British made with his own hands. A place where magic is science and evil a fading memory. A society of universal enlightenment now so close to perfection that both its creator and heroic Avator (sic) believe -- unfortunately -- that Britannia can be safe without them.
THE FORGE OF VIRTUE
Four young adventurers: Lord Ironhawk's son Jordan Hawson, beautiful Althea, a headstrong blacksmith named Drum, and Jordan's kid brother Squirt heed a mystic globe's dark warning to find Balthan, Althea's magician brother.
It is a quest that will take them through the perils of trolls, harpies, and gypsies until they discover the mission is far more crucial than they ever imagined. A vast evil has sunk its claws into Britannia. Lord British's ruling Council of Mages is in hiding, its members marked for death. And only the elusive Balthan knows the truth behind it all.