Short answers, pulled from the story.
The first Ultima game was released in 1981 with the premise of a character known as the Stranger traveling back in time to defeat the evil wizard Mondain. The Stranger used a time machine to shatter the incomplete Gem of Immortality before the wizard could fully manifest his power.
Ultima IV: Quest of the Avatar introduced a moral framework requiring players to embody eight virtues derived from Truth, Love, and Courage. These virtues included Honesty, Compassion, Valor, Justice, Sacrifice, Honor, Spirituality, and Humility to challenge players to consider the consequences of their actions.
Lord Blackthorn, the regent of the realm, corrupted the Virtues in 1988 when he was influenced by the Shadowlords. This event turned the Virtues into a rigid and oppressive code of law that the Avatar had to restore to their original meaning.
Ultima VI: The False Prophet explored the themes of racism and xenophobia through the story of the Gargoyles. These creatures were initially portrayed as enemies but were revealed to have valid reasons for their hatred of the Avatar.
The Fellowship was revealed to be an ostensibly virtuous organization that was actually a front for the evil entity known as the Guardian. The game's leader Batlin planned to create a gateway for the Guardian to enter Britannia and conquer it.
Ultima IX: Ascension revealed that the Guardian was the evil part of the Avatar himself expelled when he became the Avatar. The narrative concluded with the Avatar merging with the Guardian to destroy himself as a separate entity and stop the evil entity from conquering Britannia.