Sly Rosario was imprisoned for deceiving the gods. The poison that drips off the serpent's dagger is powerful enough to kill with a few drops, but his loyal Sierra is there to catch the lethal substance in a bowl and prevent his untimely demise. They do not speak or move, and Rosario's rage for the gods is the only thing that sustains him during his immeasurable sentence.
The lore and art is most likely describing Loki, God of Mischief, from Norse mythology, who was caught by the other gods after arranging the death of Baldr (which eventually led to Ragnarok):
"Loki was bound with the entrails of his son Nari... Skaði fastened a venomous snake over Loki's face, and from it poison dripped. Sigyn, Loki's wife, sat with him holding a basin beneath the dripping venom, yet when the basin became full, she carried the poison away; and during this time the poison dripped on to Loki, causing him to writhe with such violence that all of the earth shook from the force, resulting in what are now known as earthquakes."