„Levelling“-Dimension

I’m looking for some advice and feedback regarding this idea. This is all happening in a homebrewed universe. I‘ve had this idea of letting my players find an item which gives them access to a demiplane they can use to level up and collect resources for some better weaponry. The artifact and the dimension are relics from eras ago, when the gods would still actively choose champions and use this dimension to train them. The demiplane would house very challenging monsters and rare materials, and also some forgotten knowledge. My intention is to give them the opportunity to grow powerful and feel powerful. It would also solve another issue of mine. My BBEG is an ancient, reawakened evil, which could not be killed by the greatest heroes of its time and was instead sealed away. The dimension would give the heroes a way to grow powerful enough to defeat it, as most ordinary methods wouldn’t and (imo) shouldn’t be enough to deal with such a powerful foe. Opening the dimension requires a certain amount of gold in the form of gems, although i’m not sure if they should be able to earn these crystals from the plane itself, or if there should be more strict requirements to opening it.

I would really appreciate some feedback, if anything isn’t clear enough please ask.