I've seen a lot of people refer to the javelins as missiles, but looking closely at them in cut scenes show that they aren't being propelled by thrust. Considering that the javelin looked like it was undergoing re-entry burn, my theory is that the Javelins of Light are a form of kinetic orbital bombardment.
https://en.wikipedia.org/wiki/Kinetic_bombardment
Tl:dr is that you put a couple of telephone pole sized rods made of tungsten into orbit and drop them over your target. The acceleration from gravity ends up causing an impact comparable to a small nuke.
To me, this fits the description of the Javelins pretty well, and kind of explains a few other things;
The reason the Agarthans don't just drops these whenever they want, since they have a limited number of these things waiting in orbit, with no way to send more up without drawing attention to themselves.
The purple rings that surround the Javelin as it falls are likely some form of magical guidance system, which is what Sothis' protection around the monastery likely jammed or redirected to cause the Valley of Torment.
