When Sally and Merida clashed, it was like two natural disasters had occurred at the same time. Two demon lords, each having a more harrowing power than the other.
Sally, the Demon Lord of Rot, controlled the process of decay and breakdown of living matter, rotting. At the start, Merida thought that her body was a rotten corpse, but the very nature of her power allowed her to avoid that.
By having a demonic body, training it to the utmost limit, then breaking through all of her limits thanks to rot, Sally could exhibit the monstrous strength and speed usually seen in undead zombies without needing to become one.
On top of that, she directly controlled her rot demonic magic to wither away her enemies. Regular humans would rot away just by sensing her presence, and even powerful demons would turn into foul piles of meat in her proximity.
