In the Apple version of Wizardry I (I don't know about the other versions), the Deadly Ring has a -3 regeneration attribute (that's 'minus' 3). This means that under normal circumstances if you walk around equiped with it your hit points should go down until you die! BUT !!!! There is a computer glitch/programming issue/error/bug/whatever that prevents this feature from working.On the other hand you have the Ring of Healing and the Lord's Garb that have a regeneration attribute of +1 which heals the character's hit points while he/she is walking around with them equipped. Note: You have to actually move around in the maze, if you stand still it will not work.
To answer another one of your questions: The only item in the 'Miscellaneous' category that have an AC reduction is Werdna's Amulet for a factor of 10, but as you know, you have to give that Amulet away when you climb back to the Castle.
Snafaru