Don't be so harsh, Captain. The plot's rough and, yes, full of cliches, but nevertheless it's a plot that will take the PC's to a nice little adventure in the ruins of long lost city and so on and so on.
But still, I'd recommend editing too. Like Captain said(asked), why only a mage? Wouldn't any linquistic do? Why is the gem a portal, how was it created, by whom and why? And yes, why is the barrier between the two worlds weakening. If you could add these things, it would give the plot more realistic sense and so on.
The thing about a cliche that most people forget is the reason that it is a cliche. It is a cliche because it works, and though this isnt the best, it is certainly far from the worst. (playing good cop/bad cop...heh heh)
To be certain, it does have many cliches, from relying on the doddering old magus for the answers, to lost cities and gems as keys. Why is this cliche, because since it works, it has been overworked, like a good field not left fallow on occassion.
The question becomes how to make it fresh, how to make it good again. This can be done with a little thought, and a lot of attention to details, as they say, the Devil is in the details.
My main problem with this plot is there is not much of a hook. If the characters are of a mercenary nature, they will likely just sell the map. There has to be something to draw them in moreso than just an old scrap of paper in a bottle. Perhaps this chaos barrier is slowly failing, and the PCs are brought face to face with chaos monsters and warp demons before finding the map. Then the map becomes important, especially if the demons were powerful and hard to banish.
As always Detail detail detail
will rate after some editing Go to Comment
Pretty much agree with the content (not necessarily the tone) of what others said: Rough, needs hook, needs some telling details.
First, on detail - Don't confuse lots of detail with the telling detail. Lots of mundane detail won't actually help, it will just take up space. You need a few critical details, often tailored to your current PCs, that will allow the players to grasp the situation and their possible roles.
For example, if you have a PC with Art Appreciation, you might make the bottle interesting: "Oh, that's pre-Meduin - quite rare, because Dreanor was more or less obliterated three centuries ago, and this looks like a Priest-class double-ungent jar that would have been in use then. See, the top opens normally, but the bottom is hidden... oh, what's this?" That immediately creates context in the PC world (and makes the players happy that they took weird proficiencies).
Next, why a fishing boat? I'll guess that this is your explanation of why this has not been discovered previously - but unless your campaign is dominated by Fate or similar, the player response is likely to be "So what?" Scras is right that the apparently random discovery may or may not be important to your PCs.
Another suggestion, if you have mostly mercenary PC's who might not volunteer to "Save the World", would be to divide the information into two parts. First, give them a reasonably likely map to a gemstone in some foreign temple - get them motivated to go get it for personal gain. However, the journey turns into something very weird: Chaos monsters, etc., and maybe some bizarre effects on the PC's themselves, and chaos increases (but not yet dangerously so) as they near the gem. It's only when they get to the hiding place that they get the rest of the story - and discover that they won't be able to escape to spend their loot, because if chaos has gotten this far, it's not going to let them out.
In truth, I don't think you need to dwell too much more on plot (assuming your group is happy with a good dungeon crawl), but rather on some effects that set this apart and make it memorable. Develop the Chaos affects, the ghosts of the war (can some be made allies? - more potential plot!), and the setting, and go for it.
This post is interesting. Not in itself, but in the comments and assistance the author was given. It is a good example of the need to brainstorm with others to expand a rough idea. I have a feeling this plot could be made into a nice rpg adventure or even a book - there are lots of so-called *recognized* fantasy authors that make do with a lot less... *cough* Eddings *cough, cough* Go to Comment
"Yn these landes, theye do ryde upone greate flowtinge beastes alike as those thate ye fyshermen do calle nawtilus; Ande these beastes, callede 'pyky-pyky' because of ye noises thate the beastes make, are troubelsome ande beastlye mountes, withe fowle temperes." -Telliamed ap Ynris, "Ye Westerne Landes".
(A levitating giant nautilus that makes a noise like "piki-piki" and is thus called a Great Piki-piki.)