The term 'Underworld Boss' can refer to a variety of characters from different mythologies and cultures. In the crossword, the solutions are SATAN (5 letters), OSIRIS (6 letters), DEVIL (5 letters), CAPO (4 letters), CAP (3 letters), and DON (3 letters). SATAN is a figure in Abrahamic religions, most notably Christianity, who is seen as the embodiment of evil and the ruler of Hell. Osiris is an Egyptian god of the afterlife, the underworld, and rebirth. The DEVIL is another name for SATAN in Christianity. CAPO is an Italian word meaning 'HEAD' or 'CHIEF' and is often used to refer to a leader of a criminal organization. CAP is a slang term for a leader of a street gang. DON is a Spanish title of respect, often used to refer to a leader of a Mafia family.