I am trying to find a book, that was part of the series - I've only read the first volume and never had a chance to find the rest.
- Action takes place probably in the future, in Europe but technology is medieval again and there is some basic magic
- Main character accidentally finds the legendary Beast, which is in fact a still operational tank (although without any ammo)
- Current ruler is a typical Evil Overlord - there is a small scale rebellion that (so far unsuccessfully) tries to overthrow him, they rally behind the main character and the Beast
- I remember a village that had two magic stones: one allowed you to open any door, second was used to summon lightning.
I've read it probably in the early 90s.