Disclaimer: The characters here and the world they inhabit are the creation and property of JK Rowling and her assigns.
Snape watched in satisfaction as Pettigriew hung in thin air. He walked over and pulled out his special quill. It was difficult to write upside down, but he had practiced, and it was worth it. He tilted his head and saw that it was just right.
“Abdo!” he said. He walked down the hall, out of Pettigrew's line of sight, before releasing him to fall on the floor. Pettigrew ran up to Black, who had just reached that hallway, and tried to explain what happened.
Black shrugged. “Seriously, Wormy, there's nothing on your forehead.”
Several days later, he saw Potter and all Potter's friends eating lunch and looking at him surreptitiously. He finished his lunch and whispered, “Demonstro.”
Half the table gasped as each of the four clearly had “I am a pathetic wanker” printed on his head.
Snape laughed to himself and headed down to the lake. He needed more squid ink.
Several years later, he was out with some of his friends, doing work for his new Master. Potter and his friends accosted them in an alleyway and started hurling jinxes and hexes. Severus whispered, “Demonstro,” and was delighted to see that the squid ink was still good. His fellow Death Eaters burst into laughter, as did the few bystanders who hadn't ducked for cover. Black and Lupin looked at each other and quickly Apparated away, as Potter and Pettigrew quickly followed.
* * * * *
He stood in the shadows as the newly married Potters left their wedding celebration and whispered “Demonstro” one last time. Potter's shaggy hair covered the comment on his forehead, but nothing would cover the words, “Property of Severus Snape” on the inside of Lily's left thigh. It was an empty victory, but it was the last he would have.
A/N: Thanks to Lady Dragonsinger for the beta read!
Based upon MuseAmusant's prompt: Professor Snape discovers a unique use for the Giant Squid's ink.