Dario Bossi is a major antagonist Castlevania: Dawn of Sorrow. He is a serial arsonist who inherited Dracula's power over Hellfire and whom Celia Fortner selected, alongside Dmitrii Blinov and the primary protagonist Soma Cruz, to become the new King of the Night. He is aggressive, violent and impulsive, as well as an unrepentant criminal, but without the power, he is so proud of he appears as the weak coward he always was.
Dracula was destroyed for good by Julius Belmont in 1999 and his Demon Castle was sealed away in a solar eclipse, but Evil was not fully destroyed. Dracula was to reincarnate and a few people born the very moment of his fall inherited some of his Dark Powers. Graham Jones gained his mastery of Dark Magic, Dmitrii Blinov gained his power of Mimicry and as for Dario, he gained his power over fire and hellfire. Dario used his powers to cause many fires and became an international criminal, until being recruited by the Dark Priestess Celia Fortner in 2036.
One year prior, Graham Jones entered the Castlevania during a solar eclipse and attempted to take Dracula's vacant throne, but he was defeated by Soma Cruz, Dracula's true reincarnation. However, Soma rejected his Dark Legacy and defeated the very essence of Evil, Chaos itself. Believing that God needs the opposition of the King of the Night to maintain the balance of the world, Celia intends to make Dario, Dmitrii, and Soma Cruz himself compete within the walls of a Castlevania lookalike to see who would become the new King of the Night.
After Soma defeated Dmitrii, he confronts Dario, who rants about him defeating the other Dark Lord candidate, whom he wanted to kill by his own hand. After Soma defeats Dario, Celia barges in and teleports him away. She then summons the Fire Demon Aguni to possess Dario and vastly increases his pyrokinetic powers. If Soma defeats Dario during the rematch, he seals his foe's powers and Dario perishes, engulfed in the uncontrollable fires of Aguni, causing a bad ending.
To truly finish the game, Soma must use the soul of the Mirror Demon Paranoia to enter the mirror in the room and fight Aguni instead. Since Aguni has been linked to his powers, Dario is deprived of them now that the demon is gone and he flees the castle, never to be seen again.
Dario can teleport by turning into fire and has a fighting style similar to that of Dracula himself. He teleports around the room and attacks by hurling a fireball, by making a wave of fire erupt towards Soma, and by trapping Soma between two fire pillars before conjuring a third one right where he stands. If Soma fights him a second time, he uses Dracula's signature "Hellfire" spell (three fireballs) instead of his normal fireball and also conjures two fire dragons.
Aguni (a possible mistranslation of Agni, the name of the Fire God of the Hindu religion, and a pun with "agony") poses a much bigger threat than Dario. He uses very powerful attacks and moves erratically to disturb Soma's attack pattern. He flies off-screen to crash on the ground, causing waves of fire in either direction, breathe fireballs, and repeatedly strike the ground with his claws, each time right where Soma is standing.
Aguni is fought again in the game Castlevania: Portrait of Ruin, which is set in 1944, implying that he has been serving Dracula for quite some time.