Near the top of Rolling Ridge is a Fairy Statue.
There's a set of stairs behind that which led to Mr. Goron
The door in Veran's Castle doesn't lead to a flame delivery spot.
It is opened by finding all 10 Flames and placing them on altars, then talking to the guy in a nearby house.
He looks like Vasu and is standing next to a Crystalline Flame.
This opens that door you saw which in turn leads to an optional boss fight.
The master of combat has special requirements for activation, which aren't explicitly stated.
I could probably add a sign explaining them.
Basically, if you beat the main quest of either character and have completed all of their sidequests; this opens two optional dungeons.
Beating each of those gives you a distinctive item.
Each character must have their distinctive item to face the challenge posed by the master of combat.
And you'll probably want to do them simultaneously, since there's a switch that both characters have to press after finishing their side to get the reward.