Cerberus

Raised by Hades, this savage beast keeps the living from entering the lands of the dead and likewise prevents the dead from entering the lands of the living. Loyal to a fault to the god who raised him, Cerberus is highly intelligent and stronger than any mortal creature. Cerberus bears an unending grudge against Hercules for the hero's past exploits resulting in Cerberus' humiliation. Cerberus got his revenge, though.

History
At the height of the Titans' usurpation, the Titans pulled Gaia from her prison in Tartarus and forced her to create a being so monstrous and destructive that the gods could not stand against it. That being was named Typhon and its destructive potential only grew after it mated with another Titanic creation, Echidna. From these two vile creatures was born many of the Earth's most powerful monsters, Cerberus included.

When Hades, Zeus, and Poseidon clashed with Typhon, its mate and offspring scattered, with the exception of Cerberus which had been sedated before the battle by a clever Hades that offered the young beast an opportunity to eat from the god's hands. After Cerberus awoke to find his family scattered, Hades took the beast under his care and trained it to be a mighty battle steed for the duration of the war.

After the fall of the Titans, Hades took Cerberus with him into the Underworld. The mighty monster only grew more fierce and mighty in the land of the dad. This ferocity, when coupled with its incredible intelligence and unwavering loyalty to Hades made the beast an ideal guardian of the Gates to the Underworld.

In this position, Cerberus would roam the Underworld freely, but would most often be found strolling through the area known as Taenarum, where the six gateways from the Underworld to the Overworld were found. Here, Cerberus would scare off would-be trespassers from among the living and keep the dead spirits from deciding to return to the land of the living.

There have been few instances of beings getting past Cerberus. Notably, the hero Orpheus (in several incarnations) has shown an ability to put this beast to sleep with his magical melodies, and the mighty Hercules once defeated Cerberus and captured the giant beast, bringing him to the Overworld as proof that he had been able to defeat many of Cerberus' siblings. Hercules had planned to slay the beast after showing his capture off, but Hades himself paid a visit and threatened severe repercussions for all mankind if Cerberus was not immediately released.

Since this instance, Cerberus has had a distaste for Hercules' antics and taste for the hero's blood. Cerberus has slain three of Hercules' reincarnations, twice when the hero tried to repeat his previous Twelve Labors and once when the hero wandered too close to an Underworld Gate, letting Cerberus catch his scent.

Abilities
Typhonic Monstrosity: Born of monstrous heritage and raised by Hades, this beast now guards the Gates of the Underworld. Only the foolish or truly mighty dare confront this magnificent monster.

Gatekeeper of the Underworld: As a being created by the coupling of these two Titanic creations, Cerberus is a mightier monster than most. The beast sports three heads, each with its own personality and powers, while sharing a body of impressive strength, speed, and a skin impenetrable to mortal weapons. The process by which this beast's heads coordinate the movement of their body is flawless and fluid in execution. A true testament to the marvelous nature of this creature. Aside from a passing resemblance to a canine, much of the reason that Cerberus is often confused with such a creature is its razor sharp teeth that are able to bite through plated steel and truly impressive senses of smell and hearing, common to all three of its heads.

The Blue-Eyed Head: Cerberus' right head bears glowing blue eyes able to pierce the souls of the living and the dead alike, causing them to be paralyzed with a sense of fear and dread. Some mortals mistakenly believe this is true petrification, but that is not true. However, should a mortal be able to resist the glare of this head, they might still find themselves frozen solid by its chilling breath.

The Green-Eyed Head: Cerberus' middle head bears glowing green eyes able to see in the darkest of shadows and through the thickest fog or smoke. The eyes have also been known to be the sharpest of the eyes, discerning the smallest of details from great distances. This head also has a wicked bite, dripping with a poison that kills mortals on mere contact and causing the ground to shrivel and decay wherever it drips.

The Red-Eyed Head: Finally, Cerberus' left head has eyes the color of red hot iron and a mouth that burns like a furnace. The eyes of this head are said to be able to distinguish from the dead and its breath burns with such an intensity that not even ash survives its touch.