Cerberus

Known as the Hound of Hades, Cerberus only allowed the undead into the Underworld 3 times. The first was Heracles’ Twelfth Labor. The last – and, thus, most difficult – of the Twelve Labors of Heracles set for him by Eurystheus was bringing Cerberus up from the Underworld. Hades allowed this, but only on the condition that Heracles manages to do it without using any weapon. Even though he was bitten by Cerberus’ serpent-tail, Heracles managed to put the dog in a stranglehold and persevere long enough until Cerberus finally passed out. After this, Heracles tied him with chains and dragged the hound to Eurystheus, who was so horrified by the sight that he instantly hid in his great jar.

The second person allowed to pass was Orpheus. When Eurydice, wife of the musician Orpheus died, Orpheus went to seek his beloved in the Underworld. Cerberus wasn’t going to let him pass, but Orpheus charmed him with his music, and the hound, tamed beyond recognition, stepped aside. The final time a living mortal was allowed into the Underworld was when Aeneas visited Hades. He had some more than necessary help from the Sybil of Cumae, who threw Cerberus a honey-cake, spiced with few “drowsy essences.” Cerberus ate it and fell asleep in no time. The expression “a sop for Cerberus” originates in this story: it means quieting an uncooperative person by giving him a bribe.

Source: https://www.greekmythology.com/Myths/Creatures/Cerberus/cerberus.html