It's all implied information, but I hope it makes sense like it did to me.
page 7 wrote:
At this time, Arcanis is a dying world and those who work with Arcane Magic, Divine Magic based upon Life or Nature, or are Ardakene Elorii will feel a profound change within moments of entering this world.
Specific description for Ardekene Elorii
"No sooner do you get your feet solid beneath you that you are wracked with pain. Before your eyes, your flesh begins to shrivel and dry like a grape left out in the sun too long."
Appendix 1 wrote:
When the PCs travel to the far future of Arcanis, they will discover a world, which is dying as Oblivion comes to take into the Eternal Night.
Ardakene Elorii
Given their innate bond with the planet Arcanis, they will suffer more acutely than anyone else.
Basically the world is dying, Belisarda is the life of the planet. Therefore she must be dead for the planet to be dying.
Also Elorii can feel Belisarda presence at all times aka she is alive.
PGtA pg 24 wrote:
The immediate presence of Belisarda is felt by her priests and to a lesser extent by the common Elorii.
So what would it feel like if she was gone? Probably exactly what the Ardekene felt when they journey to that future.
Gemellus val'Assante also mentions a time called the Age of Illumination where Manetas attacked every other nation, killing the Sleeping Emperor, and had a kind of Cold War with the SK. All other races were killed, Elorii, Dwarves, Ss'ressen, etc. So it just made sense that he found Belisarda and killed her. Either knowing her identity or unknowingly. He was not the brightest bulb in the box, he was a pawn of the Silence after all.