A/N: I'm REALLY REALLY sorry for the long wait.. I was busy with classes and homework. Now I have free time, I'm going to continue this.

Disclaimer: I don't own either Undertale or PJO.

"What are you? And how do you know his name?" Sans snarled, summoning bones to attack the empousa, one of them passing right through her right leg, not doing any harm.

"W-what? Why didn't it do anything?" Sans muttered, glaring at Kelli.

Kelli laughed, her throat close to being slit by Riptide, "Your petty bones cannot harm me, skeleton."

Percy lowered his sword and looked up at Kelli, his expression calm and unreadable. "Kelli. What are you doing here?"

"To kill you, of course, son of Poseidon." Kelli spoke, looking at him innocently. Percy swung Riptide at her again, but Kelli dodged, like a martial arts master with experience. She then swung her claws at him, trying to hurt him. Percy managed to duck, only earning a small scratch on his cheek.

"Give up, Perseus Jackson." Kelli smiled, the corners of her mouth stretching to inhuman amounts. But then the smile disappeared as Percy sliced his sword through Kelli's neck and her chalk white face scrunched up in pain. Her body disappeared, and instead of her presence, a pile of golden dust was sitting on the concrete floor. Frisk's eyes widened, holding on to Sans as they shivered, a chill running down their spine. Percy turned back to them, his sword raised. Sans' eye flared blue as he stepped in front of Frisk.

"Don't hurt the kid."

Percy raised his eyebrow, looking at Sans weirdly. "And why would I do that?" Riptide shrank into a normal ballpoint pen as Percy put the cap on.

"And what's with all the 'Son of Poseidon' crap?" Sans growled, his face full of suspicion.

"It's not what you-" Percy started, to say, his eyes wide with worry, but his sentence was cut short by Sans.

"Kid," Sans narrowed his eye sockets, "you have a lot of explaining to do."

A/N: Btw, sorry for the crappy fight scenes.. I tried my best..