Well, it really sucks to draw something up, put out there on this forum for the world to see, and then have nobody post. You get a sense of feeling that everyone is doing the whole 'if you can't say anything nice thing'. And it's a little depressing, even if you did just pump something out in 20 minutes. I know it felt a little bad for me and my lizard hippo thing, even though now I'm starting to think that perhaps he's just not meant to be. But lemme put that aside and talk about your giant dragon fellah here.
Let me start by saying, I'm not much a fan of huge meta backstories. To me, small scale things have always been better, just because there more relatable. Basically, the bigger and grander things get, the bigger and grander everything has to become to keep pace and it can get very silly, very quickly. An example would be Dragon Ball Z, once the protagonist is powerful enough to destroy a planet, the next conflict that arises must be -stronger- than that, and the next conflict must be stronger than that still, becoming very over the top, very quickly. Which is okay for Dragon Ball Z/GT, the writer runs with it and it works. And also, I'm not comparing your writen piece to Dragon Ball Z directly as it's no where near the same scale, but using DBZ as an example of how scaling can everything from the plot, to the world setting, to everything.
But I'm prattling on now, though, I just wanted to give you some insight into my believes, because they are the believes that will shape my reaction to your work. Though, despite how I have outlined why I might be predisposed to see your work in a negative light, I'm actually going to focus more on all the positive things it brings to the table.
For one, the idea of a boss creature such as this being present in the game, and unique rewards being available for his defeat are very much what I feel something going in the right direction. For example, the horn you mentioned being important in the fusing of certain magics, for example, is a very good example of that sort of mechanic. I can imagine alters found in the game where players much gather certain things in order to summon a creature similar to this, then, need to overpower and destroy it in order to recieve the horn, and other valuables that would be unavailable anywhere else.
Though, I might mention, if his horn is important in that way, it makes it more likely there is more than one of this creature, otherwise, how would anyone know the horn achieves such a thing? This lends me to think that this fellah would serve better as a sort of 'there's more than one, but they are a very powerful and rare' sort of deal. This makes the idea of the horn being covetted by wizardy types much more. Perhaps the great purge mentioned might have involved a war with a spieces of this type, rather than a lone, legendary figure. Perhaps a few were slayn? And this is how the magical properties of the horn were discovered? I know you mention an alternative of their being bits of the horn scattered underground and that provided the insight into there properties, and that could still be implemented.
Also, let me mention the scale of the beast. Something the game, as it stands now, could in no way support. But, that's actually cool, because I would like to see the game -become- capable of supporting a creature of this scale. I'd like to see the implementation of the depth digging system to become more complex, for players to be able to dig deeper, to be able to have rooms under rooms, to make thier dungeon 3 dimensional in ways the game doesn't support right now. I would actually like to see this game take some ideas from Terraria and Minecraft. Let us really get into the whole leggo building thing. It might be hard to do, but very cool.
Anyway, It's getting late, so, I'mma stop here for now. Rep for you for putting yourself out there.