Now, I've been up against some scary things -- Shabranigdo, Phibrizzo, Gaav and Valgaav. Not to mention various dragons, bandits, sorcerers, and a hungry Gourry.
But the scariest thing I've seen wasn't something I've fought, thank goodness. It was the Lord of Nightmares, in my body, crushing Hellmaster Phibrizzo like I might swat at a mosquito, after he had attacked me while I cast the Giga Slave.
At least the mazoku acknowledge my existence while trying to kill me. She didn't even seem to notice what she was swatting, let alone me.