I've written some lore for a horror campaign I've been doing, involving flesh monsters and a parasitic hive-mind. So far, my party hasn't discovered too much, but I'm hoping to include some locations from the lore... I'm just not sure if this lore would work well.

In a time that faded into obscurity ages ago, there existed a prosperous nation of humanoids that were the ancestors to all the humanoid species of that world in the present. However, these Ancients were startled by the sudden arrival of what they first thought was a meteor that landed not too far from their capital city: When they went to investigate, they found an enormous being in the center of the crater, injured and weak, a creature they would refer to as the Defender. Believing this creature to be a god, they were kind to the great being, and it formed a bond with the people of this world.

What none of the Ancients knew was that this being was one of several powerful beings that have been roaming the universe, proclaiming themselves to be gods, whose enormous interstellar ships required the metal cores of planets and the hearts of stars to power. The Defender was initially sent to this planet to destroy it so they could claim its still-molten core, but it noticed that the life on this world was intelligent, and wanted to repay their kindness.

The Defender's superiors noticed that they were defecting, and sent some of their own creations to 'hasten the process', but the Defender valiantly protected the Ancients and their world. The Great Fire Worm was cut to ribbons, The Storm Dragon was hastily dispatched, and not even the Moon Beast could rival the Defender's strength. Eventually, another of the Defender's kind went to the planet to finish the job themselves, and a great battle ensued. The Marauder's head was cut from their body, and the day was saved once more, the crystal in the staff the Marauder carried shattered into pieces that would power the Ancient's technology.

The 'Gods', seeing that they lost one of their own in this futile venture, left that part of the cosmos, with the Defender to live in exile on the planet they fought protected for as long as they live. This sent the Defender into a deep depression: Not only had they killed one of their siblings in the name of their cause, but they would never be able to communicate with another being again. They left to a deep cavern under the mountains to mourn, while the Ancients celebrated their victory. Eventually, after hundreds of years, something inside the Defender changed: Something snapped.

The Ancients awoke to the horror of the Defender, the benevolent being that protected them time and again, slaughtering millions and ravaging entire cities within hours in a mindless sobbing rage. The Ancients scrambled to procure enough people and weapons to fight against them, and ultimately succeeded, but only a few hundred remained from what was once tens of millions. They buried the Defender deep within that mountain cave, and destroyed almost all records they had of the time that they were once allied with the it, carrying their grudge to their graves as their once-splendorous cities decayed around them. The grief of the few that remained was so strong that, intermingled with the restless spirits of millions and the sheer power emanating from the Defender's corpse, created a permanent rainstorm over the mountains, which have since eroded into a lonely plateau.

But the Defender never truly died. Its newfound hatred and malice persevered to such a degree that the being simply refused to perish. From its own decaying body, the Defender grew roots and veins that plunged deep beneath the earth, birthing the parasitic infection that is the Redroot, and the horrid flesh monsters that now plague the kingdom formed around that lonely plateau.

TL-DR: An ancient people found a powerful being that was injured, and helped it. The being ignored its directive to destroy this world, and instead fought to protect it, even going as far as to kill one of its own kind. Realizing what it has done, the being goes into voluntary exile, only to return centuries later on a mindless murder rampage. The Ancients manage to stop it, but their losses are so great that their kingdom falls into ruin shortly after. Through sheer force of will, the dead being extends its influence from beyond the grave, creating a parasitic infection that threatens to ravage the newly-recovered world.

Gosh, that was a lot. Does any of this make sense, or am I seriously overthinking this?

The tldr is a very good set up and is all you need in order to drop lore in large generalizing sweeps. However, the rest is good if only to have names for thins when inevitably asked about them. 

Just a small clarification - if only a few hundred remained, and the Redroot began to take effect right after the battle, then that's effectively a destroyed race, not one that comes back to flourish once more with a civilization.