Ultima V, on the heels of the moral and technological breakthrough Ultima IV, has that King Arthur quality to it. It's been 2 years in your world(20 in Britannia) and someone named Blackthorn has usurped the throne after Lord British has disappeared. LB has been spotted at marshes some times although Blackthorn has persecuted any who say so as sacreligious. While LB was ruling the Avatar's eight virtues were expounded and practiced to make Britannia more wealthy, but now there are high taxes and martial law and Blackthorn has twisted the eight laws to form a police state. He even wants to figure out the mantras and the secrets to the Codex and the now sealed-off dungeons. Your object is to rid Britannia of the menace he and possible accomplices pose to Britannia.
Iolo and Shamino realize the kingdom is trouble and call on the Avatar of legend to come back. He does, but as he is summoned through the magic moongate three Shadowlords appear and one throws a bolt into Shamino, who is left on the edge of death. The three heroes retreat to Iolo's hut deep in the woods, where they hope to let Shamino recuperate.
You'll need to get Lord British's artifacts, the Sandalwood Box, visit eight shrines and solve their quests (need to get the mantras too), get eight words of power to open the dungeon, and find Lord British. You begin as an avatar, but can lose the status by harming virtues. In the game this loss of Avatar status is shown by important clues being withheld. For example, in Britain, you won’t receive the password for opening the dungeon Despise if you haven't followed the path of the Avatar. The game now also has day and night cycles. In the night, the gates to most towns are closed and the people living there mostly lay in their beds although guards still patrol.
One of the new improvements is the magic and combat system. In a combat situation you have a cursor which you can use to attack in any direction. That cursor will remain on the selected monster as long as you wish. That doesn’t mean combat is easy since you only get 30 hit-points, more when a new character level has been achieved, while monster magic capabilities and sheer numbers have increased. Magical spells are now (and in further Ultimas too) divided into 8 cycles. The higher the level of the spell caster, the more magic they can use.
The game is available at gog.com and is part of the "Ultima 4+5+6" package.