Your master was Cerberus. The giant three headed ancient Greek guard dog of the underworld. In charge of making sure no souls that don’t belong get in or out. She was roughly 300 feet tall give or take, and covered in black fur. All three of her heads could talk, but the one in the middle seamed to be the main one with the most intelligence and control over the body. The other two behaved more or less like dogs, but occasionally spoke a few sentences.
When the change first happened she was rather confused. Having memories of guarding the gates to hades for centuries, yet also her time as your penis for some years. She had much to think about, spending most of her time trying to come to terms with her memories. The four of you had woken up in a cave Cerberus recognized as her home, with the gate to the underworld behind her. After some thinking, she told you that she actually rather enjoyed her time as your dick, particularly how often you used her finding it some of the best memories of her life. Vowing to use you for mutual pleasure just as much as you used her.
So she tasked her left head with pleasuring you whenever her or you got in the mood. As a result the left head got VERY good at licking you clean and eating you out. Eventually the right head got jealous and wanted a taist of you. The two heads burry their noses in you, licking your pink insides until you couldn’t take it anymore and discharged a wave of dog lady cum. Only for the other head to join in often immediately after.
So things went for some time. While the main head kept watch. The other two would take turns masturbating. Cerberus’s main head pretended not to care, but you could tell from her tail wags and the way she licked the discharge off her fellow heads muzzles she was having fun to.
After a few months of this Cerberus finally said that she hadn’t noticed ANY souls come by her at all. Which was strange. So, without further ado the guard dog of hell stood up and went to investigate the lack of souls going to the underworld…