So, I just realized that we, the 'World Guardian' and hero of Runescape, ultimately caused the apocalypse and annihilation of most of Gielinor's inhabitants in Dimension of the Damned.
How? The promo poster hints at our death, and that we shouldn't have talked to an elder god "that way". The massive destruction and undead hint that this elder god that we angered could be the undiscovered 6th god that Zaros warned us about in The Light Within.