Fallen Phoenix Dust

"Monsters of the Fallenrealm used to be humans with magical powers," someone had once said.
They were changed into monsters when they were banished underground after being betrayed by one of their own.
Though, only the monsters themselves know of that tale, no one else does.
Since then, this world had one ultimate rule: betray others or get betrayed yourself.

Since then, the Fallenrealm has been facing an endless cycle of insanity as the 8th Fallen Human, Frisk, along with Chara, whom shared the body of Frisk, kills almost everyone in the Fallenrealm except for one single monster.
Everyone remembers when they were the lone survivor of the Underground, but not when they died.
Paranoia becomes obsession to live and save others, but soon enough they give up and give into their insanity and desire to live, no longer trying to save one another unless they were important to them.
Several humans have fell since these "Genocidal Runs" began, however, they never made it out to the surface...