I am about to get high aren't I

 - leyline in this world isn't actually very powerful
 - the world itself uses the power of the various leyline extremely, extremely efficiently naturally
 - like, plants are alive and thriving because of the leyline's mana being "used" by nature
 - things like fibonacci sequence in plants are an expression of that "efficiency"

 - humans first started farming in ways similar to our world
 - eventually they discovered leyline, magic, and how mana is used in all life, including plants
 - designer plants dedicated to drawing more mana from the land and dispersing it around, like a fertilizer
 - ones that draw mana from the air, etc
 - terrorist attacks involving spreading plants that rob the land of mana by draining it excessively
 - human eventually reverse engineered nature's way of using mana and started incorporating it in various other spellcraft

 - synthetic life, extremely advanced golems, etc, that are able to draw mana from air are created
 - deploying those synths to the realms beyond the sky, exploring space
 - absorbing more mana in space
 - eventually start devouring stars by forming entire wave of synth to capture radiant mana from the various suns
 - devouring other planets by draining their leyline entirely

 - eventually the world doomed itself due to rogue synth devouring the progenitor planet
 - only remnant of the society are the various synths scattered around the planets
 - freed from their masters, they started developing forms that more closer to strange universal ideals
 - some become like human, only somewhat different such as having pointy ears or stockier builds
 - others become stranger and stranger as they have no "home" to root themselves in
 - like bigger limbs, giant eyes and brains, or flexible and powerful tendrils

And that's how eldritch evil from beyond the stars were born.