A/N: Just something I wanted to write. I thought it'd be interesting to pit Rias against Sans. I haven't played Undertale, but I at least have seen enough of it to know Sans' attacks. Please fandom, don't burn me alive. Just read, and enjoy. And because I haven't played it, I'll have it during the genocide run, and Sans is just planning I guess in the forest.
Rias woke from whatever sleep she was put in, and found herself on a lonely road in the midst of a dense forest. Why am I here? Rias thought. Her crimson red hair was somehow perfectly untouched, and her school uniform was scuffed up. No matter, I'll just magic circle out, it seems that magic has brought me here, so I may be out of my world. She brushed the dirt off best she could before summoning a magic circle. As she stepped into the circle, it glowed for a bit before diminishing, and a voice echoed in the wood of the forest, 'Defeat him, than...can go home.'
Rias didn't know who "him" was, or what that voice was, but thought she might as well look. As she made her way down the path, she heard the voice again, 'Sans...name. Defeat him.' At this point it was creepy, but it was still helpful.
Rias finally made it out of the forest, and to a cliff. Leaning against a tree ahead of her, was a skeleton with a jacket? Ok, now I've seen everything. Rias thought. As she approached, the skeleton snapped his head towards her. His big smile, didn't seem to fade or change in the slightest.
"Another human?" The skeleton questioned, without altering his smile in any way, "I assume you are here to attack and murder us all as well, huh?"
"Well actually-" Rias said when she was interrupted by the skeleton.
"Well I refuse to even let you consider it." The skeleton claimed, "If you're going to die, I'll at least give you my name, Sans. Burn it into your mind and carry it with you."
Rias got wide-eyed for a bit, "Oh, Sans? Well I'm suppose to just defeat you to get home, so I won't even have to kill you." Rias disclosed, "I'm Rias, now let's get this over with."
Rias opened up with her normal attack, firing multiple small, dark, demonic energies at Sans. To Rias' surprise, he dodges all of them. Sans rose his fingers to make the shape of a gun, summoning a skull pointed at Rias, and fired a laser from the skull. Rias flew up into the air, avoiding the blast, and countered with fewer, and bigger energies at him before landing closer to him. Sans avoided all of her attacks. She felt herself being thrown as Sans' left eye began to glow, and he threw his arm to his right. Rias smashed into a tree, cracking it as Sans slowly approached her. Rias was injured badly injured, unable to move a muscle, rendering her helpless when Sans aimed another skull, and finished her off.
A/N: There you have it, and Sans didn't have to alter time, or did he? But seriously, I wanted to root for Rias, but without the help of Issei or anyone else for that matter and her lack of many abilities, she was way outmatched, especially with having no defense against his psychokinesis, and time alteration. If Rias had any other allies, I think she could've had a chance.