SolvedHow-toUpdated
Queen's Domain: Upgrade Your Strange Stone
Short answer
Change your boring rock cube into an electric explosion of death.
Steam Community guide by AfroRyan.
Step One: Square Peg Square Hole
Throw your cube here. You'll know you got it right if a purple flame shows up above the diamond shape that's between the stairs (spoilers: it's a door).
Step 2: Do it again
Now throw your cube here. This is the trickiest shot. I don't think the order actually matters, but this is the order I did it.
Step 3: Almost There...
Last one. Sink the shot, and now check the diamond shape between the stairs.
Step 4: Kick Some Butt
The secret door will open, and a secret boss will come out. Don't worry; he's pretty easy. It's a larger than normal gargoyle and it's also mechanical. Beat him up, add him to your bestiary, and then descend the stairs to upgrade your boring rock into an electric bomb.
NOTE: The rock may be boring...but now that it's upgraded, if you revisit the merchant, you can re-buy the rock and stack up to 8 (!) of them, so even though the rock isn't as exciting, with 8 of them, you're going to have no problem stun-locking enemies by throwing them off balance. Anyways, have fun!