For years, humanity and nephilim have battled (more or less) together to staunch the unending flow of demons and angels wrecking havoc on Earth in their unending wars.
Now, we are finally in position to strike back. By creating the Collegium, we shared our knowledge in arcana and xenobiology and feel confident in our capacity to repel those extraplanar warmongers.
Our Great Marshal tasked (among other things) the Collegium to design a spell to let our allies over the world know when to launch the attack. Through inner politics and games of influence, the task was finally assigned to my team of arcanists.
The thing is, I would have preferred if the GM had a little less flair for the dramatic, as he described the signal as "the day the sky will burn". The main reason, aside the psychological effect, is that the angels monitor Earth from the sky, and impeding their intel could prove crucial in our effort. We are free to come up with any solutions we want, but the prerequisites are:
- It has to be visible from far away. If we can not cover the Earth in a single go, we can arrange for other arcanists to relay the signal by casting the same spell. Around 10 teams of arcanist, ourselves included, could participate in that effort (the fewer, the better).
- For tactical reasons, this attack will be launched during the night from where the order is given. Admittedly, the light pollution on the sunny side will probably deteriorate the visual signal, but we need to achieve maximum coverage and will make do with sunny areas not seeing the signal.
- The sky does not have to be "ablaze". We concluded that putting the sky on fire could prove harmful to our own survival. Any kind of visual effect, like auroras, that can fill the bill are allowed (color is irrelevant).
- My team and I are quite competent, and with enough preparation, could achieve the same energy output as the famed blast of Hiroshima (around 15kT). But the less energy we spend, the better we will fare for the battle that will follow. Our effective range is about 1000km, as long as we can achieve line of sight (no physical obstacle between the emission point and the target). 1
- The effect has to last as long as possible, as it will hide Earth's surface to angels eyes, but we need first and foremost to send a signal. Any duration our sentry around the world will be able to reliably perceive will be good. A few dozens of seconds could go a long way in striking the enemy before they can coordinate.
This is why I gathered you, our specialists, to browse the archives. We need you to look through our tomes about physics and chemistry, as they were passed to us by our ancestors of the 21st millennium. The solution requiring the fewer arcanists and power spent shall be favoured. If it also could be as harmless as possible to Earth, it would be preferable. (Certainly nothing that would kill us all shortly afterwards).
Order of importance is: number of arcanists team > power > harmful to Earth's habitability by humans > duration of the effect.
1: Any kind of reaction we know how to theoretically achieve today, in the energy limit stated above, can be used.
PS: Don't hesitate to ask for details about limitations of magic, as my answer will be highly dependent of what you need to know.