The accessory would give you:
- 8% reduced mana usage
- Automatically use mana potions when you need them
- Increased pickup range for mana stars
- Causes stars to fall when you take damage, which harm enemies and can be picked up to restore some mana
- Enemies are less likely to target you
And if we added the Celestial Cuffs, you would also get +20 max mana and some mana restoration when you get hit.
To be totally honest, if it's late enough in the game, I don't think this would very OP. It's a lot of benefits, but mana isn't all that hard to come by anyway, and a Mana Regen potion would probably be more effective than this if you're playing carefully. Someone with better knowledge of the game's balance than me could probably weigh in, but I kind of like it!