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I'll just say a couple things that you may want to take into consideration:
*What's up with the changing caster level? The stated caster level is 10th for the 1/day effects, many of which are what the SRD has as 9th 8th and 9th level spells (note the wording there, since I do not think ''meteor swarm'' and its ilk in any way constitute 9th level spells) which would not be useable at said caster level. Then there is a caster level 20 for things like fireball which don't even give a lot of power. All in all, I find most of the powers that this item grants pretty weak when I imagine it as an artifact. ''Fireball'' and ''cone of cold'' are practically 1st level spells in power. No joke. So you may want to consider some other options (''[[SRD:Fire Storm|fire storm]]'' isn't a bad choice and there are quite a few solid homebrew effects if you are open to using those) and maybe set the caster level to a default 20 no matter what the effect is. This really isn't giving that huge of a power boost; it's just making it fair across levels and actually conceivable that the horn could create a level 9 spell.
*On that note, something as strong as an artifact should allow the user to control the elementals he summons (or, at the very least, prevent them from attacking him and his allies). As written, this could go from being a cool item to your party's downfall. And then it will never, ever be used again by that party and it will have lost all meaning if there was much risk of summoning a group of elementals who wreck them when they were hoping for something helpful. That inclusion is one of the reasons why I'm not a huge fan of chance; taking too many variables out of the players' control rarely enhances the game experience (unless by 'enhances', you mean 'cheapens' and 'demeans'). It just leads to too much frustration to be useful. That said, I would either improve the user's defenses against the effects or improve their control over it. Sure, it doesn't go well with your fluff, but fluff text is mutable.
