The Phlegethon, river of fire, churns through the bowels of Hell, boiling the souls of the damned. As it coils down to the deepest depths of Hell it approaches that prison of the titans, dreadful Tartarus. The great guardian beast Cerberus patrols this river as well as it travels the borders of the underworld.
"More dreadful things have I seen which Phlegethon bids imprisoned sinners suffer, compassing them about with his stream of fire; what punishment waits for me, and what place, I know." Seneca, Phaedra 1226 (trans. Miller)
"Smoky Phlegethon rolls down his streams of murky flame, and Styx interfluent sets a barrier to the sundered ghosts." Statius, Thebaid 4. 520 ff
“Aeneas looks back, and under a cliff on the left sees a broad castle, girt with triple wall and encircled with a rushing flood of torrent flames--Tartarean Phlegethon, that rolls along thundering rocks. In front stands a huge gate, and pillars of solid adamant, that no might of man, nay, not even the sons of heaven, could uproot in war; there stands an iron tower, soaring high, and Tisiphone, sitting girt with bloody pall, keeps sleepless watch over the portal night and day." Virgil, Aeneid 6. 548 ff (trans. Fairclough)
Cerberus, three-mouthed, guards the borders of Hell. The dead may enter his borders freely but any attempting escape must face his vicious maw.
"Once, they say, the gate-wrecking, unconquerable son of thunder-flashing Zeus went down to the house of slender-ankled Persephone to fetch up to the light from Hades the jagged-toothed Cerberus, son of unapproachable Echidna.” Bacchylides, Fragment 5 (trans. Campbell, Vol. Greek Lyric IV)
"And before them a dreaded hound, on watch, who has no pity, but a vile stratagem: as people go in he fawns on all… but he will not let them go back out, but lies in wait for them and eats them up, when he catches any going back through the gates." Hesiod, Theogony 769 ff
Awesome atmosphere created by those fires and chains. I'm not the biggest fan of the micro-cerberus, cause it's kind of necessary to see the big one for the smaller one to make sense, but I do love the innovative concept, as opposed to the classic 3-headed dog.