True Chaostale is an Alternate Dimension where Asgore absorbed the human souls and the History of the characters are changed (head canon)
True!Chaos Sans is a moderator character.
At the end of the genocide run he makes an interesting decision, he chooses to not fight Frisk in the Judgement hall.
True! ChaosSans Plan- He wants the human souls but Asgore had already absorbed them so waits silently in the darkness for the demon to distract Asgore, and Frisk unknowing obliges.
Now with Asgore's full focus on Frisk, He takes his opportunity to strike and stabs his boney hand into Asgore's back, paralyzing him completely. Now with Asgore out of commission he begins to drain the human souls into himself.
What will the souls do? As he absorbs the souls his body begins to crack under the pressure of such power, but something interesting happens, his black soul swallows up the 6 other souls infusing him with power that rivals the God of Hyperdeath Asriel.
And with his newfound power, he takes away Frisk's ability to reset and grabs ahold of their heart, leaving Frisk's body lifeless. He absorbs a soul full of hate, and LOVE which in turn heals his body and staining his bones a dark gray, and black.
What does he do after killing the child? Simple with his newfound godly power he travels across the entire Undertale Multiverse doing whatever he wants.
"You want the full story go check it out on Wattpad by Peyton Swaney
Deviant Art- ChaosSans