Thanks for your answer.
Obviously I can always adapt the Khoras Canon, but I prefer to stay in it (or argue to change it if I have arguements).
I even think it's better to imagine a worldwide known Focusing, as every one on Khoras known WHO is responsible of this disaster. It's easier to explain the Mage Lord hunting, and it's a better "humility lesson" from gods. Nevertheless, it does not mean tat every commoner, even in Aden, know exactly how it works and what is the Focusing schedule. But I guess everyone there know the final goal of the Mages.
My players are not used to read the website. I did not forbid them nor encourage them to read it, but they know that it would somewhat "kill the mystery". They know what their character knows and that's fine. However, it seems to me that one of them did read some part of the website. Anyway, they all know the principle of the Sundering, the Alliance Mage Lords and the Thullian Empire.
I think I will be obliged to use some tricks :
- replace names with former names : the Thullian Empire and Traxx Legion will be called the Miratz Empire and Kingdom of Gekron.
- present a different type of Focusing spell : something to change the magic in the Empire (why not a magical protection for the whole empire, a way to change the magical field, etc...)
- place the event in a neutral period (and say it clear) to avoid questions about Drellis
I have already presented them they will play on a different continent (hey, Ithria was very different at this time, wasn't it ?), with an organisation of very potent arch-mages. I guess they imagine playing on a fourth continent (we have already played on Queshi and Agraddar), kept secret because quite far or different. So a very powerful mage organisation in 2700 C.C on a remote continent is a reasonnable assumption for them.

The adventure will take place partially in the Assembly Hall, but is not focused on the spell itself, so I guess the trick will work. As soon as they will understand what is happening, I will reveal the truth.
I will change some things on the Focusing/Sundering events, and I have arguments for it.
It is usually admit the magic does not travel very well through matter. Tens ot hundreds of meters of matter are enough to stop most spells. Even if Alliance Mage Lords were able to enhance the spell penetration through matter, it is impossible to be enough to reach the inner layer of a star (thousand of time the "thickness" of a whole planet). So I prefer to imagine they send the magical energy with an interdimensionnal portal directly inside Drellikar, rather than letting the magical energy travelling through space and drilling through the star.
When the Sundering begun, a portal was open to send magical energy (to try to reverse what appeared to be an immense mistake). A tiny part of the huge sundering energy overcame the portal protection, and Drellikar energy came directly on Khoras surface, wiping out the Aden entire region. Only 8-9 minutes later came the brillant flash and so on.
As the players came back to Tosche, via a small scale portal, they are safe from the Aden events. But the energy also goes through this second portal to make the mage tower explode. Few minutes later, the shock wave of the Aden region reach Tosch and make violent earthquakes. Then come the flash which severly burn every square meter exposed to the direct light. Magic is gone, giant dust storms, eartquakes again, burning rocks falling from the sky (from Aden)... Exciting program !

I will post a description of the sundering events after the game.