Cerberus rolled his many eyes and huffed. Guarding the Underworld was a dull and tedious job, however important, and no living men dared venture there. Part of the blame went to his sister the Hydra, for she took any living men and killed them where they stood. He thought about going up to visit her, but was at loathe to swim across the lake or walk through the never ending tide of souls drifting by. Hades fed him now and then but he still did not wish to stay here for eternity, it was so boring. He wasn't allowed to lie down, or he'd do so now, his legs ached so much from standing up for the last few decades, but if Hades caught him lying down, he'd be punished for at least a century, that's what happened last time.
Maybe if he was lucky, a brave mortal may venture there and give him some entertainment, but it was such a long shot that he discarded the idea completely, training his eyes on the entrance, longing for change to his monotonous existence in the entrance to the Underworld. He watched Hades as he returned from Olympus, and growled his greeting. The lord of death gave him no acknowledgement other than to throw a steak at his feet as he passed on his boat, which angered him. How dare he show him such disrespect? The boat disappeared into the shadows, and he was left alone once more, bored, furious, and plotting. Having three heads gave him three times the brainpower, and three times the thinking speed. It would have taken a one headed monster decades to make a decision as complex as this, but he was done within hours.
Grinning, he swallowed the steak, needing sustenance, and trying not to show his disgust, slipped into the river of souls. It felt strange, but he swam to the entrance, where water flowed, and took a deep breath, before ducking all three heads under the water and swimming strongly towards the Mortal World above. It took him ages and ages to swim through the underwater cave, and he did it without taking one breath, after all, there was nothing to breathe but water anyway.
He finally saw light, for the first time in centuries, and gladly broke the surface of the lake, to the shock of his sister. His fur gleaming an oil black, he took himself to the bank, shook himself dry, and, ignoring the Hydra's confused questions and protests, Cerberus walked away from the Underworld, done with guard duty, to find some entertainment.