I guess, just for funsies, I'll explain my headcanons of what the Mark of Mastery is for and how you get it and blah blah blah. At this point, who can say what is canon or not in what I'm about to say, it's all speculation.
The Mark of Mastery is not much of anything except a formality and it grants new Master permissions and knowledge they wouldn't get otherwise.
Since this was never delved deep into in Birth by Sleep, we can't say exactly what special info or permissions a Master might have other than a few
I can speculate:
- You are granted special knowledge about the history of the Keyblade and wielders (knowledge that Aqua, the only master we've followed closely, did not get the opportunity to explore?)
- If you are chosen as successor, you learn very important things like how to protect places such as Land of Departure (that may have a special significance we do not yet know of)
- You are allowed to grant new wielders the ability to one day wield a keyblade if they are deemed worthy (Xehanort did technically name Terra a Master. The only one who didn't pass on anything was Ven who was certainly not named Master in any way... so far)
As for how you can become a Master, I'm pretty sure it's stated that the Master gets to decide how their pupils can gain the title. It's totally up to them on what absurd trials they put their students through. It could be as easy as just saying, "Congrats lad, you're a Master now!" or even making them go through a hot dog contest and the biggest eater wins.
In Eraqus's case, his decision on whether his pupils would become Master's or not is if they didn't show signs of darkness, using darkness, or struggling with darkness. Terra failed that test. I'm not saying Eraqus is absolutely right though, Eraqus is the one that made Terra so insecure about his darkness in the first place. Instead of telling Terra everyone has darkness and helped him find a way to balance it out or control it, he insisted Terra just push it down and pretend it didn't exist.
That said, Terra did fail Eraqus's test, and as we saw, Terra did have problems with his darkness with unfortunately lead to the bad guy manipulating him into losing his own body.
This parallels greatly with Yen Sid's test with Sora and Riku. Yen Sid's only requirement was for them to return safely and learn a new power and they would be granted the title of Master. His test was seemingly even easier to pass than Eraqus's. Sadly, due to Xehanort, Sora did not return safely at all. It's not Sora's fault at all, but he didn't meet the requirements AND Yen Sid said his grasp on the power he set out to learn was sorely lacking.
So, to become a Master you must:
- Have a steady heart and have confidence in your own balance (mostly). By the time DDD ended, Riku had battled it out with himself one last time and is confident in his balance of light and dark. Aqua is very confident in her balance of light and dark and even if she faltered once, she still wasn't anywhere near actually falling into darkness.
- Pass the specific test each Master puts forth. It's up to them to decide. If they hate you, maybe you'll never pass despite having proven yourself countless times or being the strongest person ever
And lastly, for the tests itself, specifically the one in DDD. What happened in DDD was a very special test. Yen Sid was trying to come up with a way to surprise Xehanort, so he set Sora and Riku off on the down-low trying to help them gain a new unexpected power that would turn the tides. Unfortunately, Mr. Mad Man Xehanort always has a plan and seems to be 5 steps ahead of Yen Sid somehow.
I think what Yen Sid did was fine. Some people roll their eyes at Sora and Riku starting over, but I think it was a clever way to say, "Hey, you were doing fine before, but you can be even better if you review the basics properly." or even "Hey, I know your techniques are cool and all, but if you learn it in this special way from the beginning, all your techniques will have a new flavor to them, making them even better."
Technically Sora lost all his strength, but we saw that he at least retained some basic stuff like magic and even flowmotion (and new stuff that is clearly a new take on shotlocks). His training (even though he failed the test and was stripped of most everything) still added a new 'flavor' to his existing abilities thanks to him starting over in DDD. That's how I look at it anyway.
To summarize, the Mark of Mastery and becoming a Master is like... driving licenses. Every keyblade wielder can have a license, and some, like Sora, are 100x better than all the other wielders. BUTTTTT.... if Sora can't pass the written exam to get a commercial license, then Sora's can't drive a big rig across the country to deliver cookies to a bakery no matter how good a driver he is. Not every test is based on your skill.
(fyi, I almost failed the written exam to my normal driving test. and I hit the curb on the actual driving test. But I still have a license so. I don't think they care about keeping terrible drivers off the road. Hahaha)