All right, he thought, no more Mr. Nice Bastard.
He twisted himself around and pointed his wand at the advancing Snape. "Y'shouldn't've done that," he said with another unholy grin. Without looking, John pointed his wand at the ground and snapped out the first spell to come to mind- "Reducto."
Dirt sprayed in every direction. It wasn't much of a distraction, but it'd do for what he had in mind- with a little help. He didn't have time to amplify his voice, but with any luck the words alone would be enough.
He drew a deep breath and got hold of the knucks in his pocket. "Dia ad aghaidh 's ad aodann," he intoned, starting to rise with his good leg. "Agus bas dunach ort! Dhonas 's dholas ort-"
The dust cleared. Snape was staring at him with an utterly baffled expression from barely a yard away. Just like he wanted.
"Agus leat-sa!" John bellowed-
There was a sudden, sharp smell of ozone, and the air between John and Snape shimmered with a horrid distortion. John fell back, flinging both arms up in front of his face. The magically paralyzed leg screamed in pain as he hit the ground again; he tapped it with his wand as quickly as he dared, murmuring the releasing spell under his breath.
"What did you just do?" cried Snape, who was staring at his hands and his wand with an expression of utter horror. "What spell was that?"
But it wasn't a spell, John thought. It was just a line out of Lovecraft! Not even one of the bits with blasphemous names in. Just some bloke going mad, the way they always did, only he started blithering in Gaelic before he lost it completely. What the hell had he just done?