Ring of reminding
Simple plain steel ring that feels warm to the touch. It's pretty nondescript.
If you put in on it's warmth will constantly remind you you're wearing it.
If you put it on a finger of another person, the wearer can not remove it.
The ring then gives just enough fire damage not to give 1 HP of damage. As a constant fiery reminder to the wearer where it's loyalty (should) lie.