Ocellus is an insane Guardian of Guthix who fundamentally misunderstands Guthix's teachings. He is currently trying to stop the Order of Ascension, an order he created in an attempt to create beings who would not worship gods. However, they ended up following Guthix. After Guthix's death, they went rogue and attempted to ascend a man to godhood in order to create a new Guthix for them to worship. Ocellus tried to confront them but, after failing to stop them, he trapped them inside their monastery and stands guard to ensure they do not escape. Ocellus and blood reavers are the only examples of a race known as the virii, a type of demon. He used to be Zaros' pet virius in the Second Age, along with his brother Auriculus.
When asked why he wasn't with the other Guardians in Chamber of Guthix, he will state he had important work to do, and that Guthix needed work, not bodyguards. However, his journal reveals that he wasn't even asked to join the other guardians, likely because the other guardians don't like or trust him. 
When he was made a Guardian of Guthix after the God Wars, he was one of the few to know and understand Guthix's wish of not being worshipped. He was not given a particular task, however. Hoping to create humans who would be free and not worship any gods, he founded The Order of Ascension. After failing many times, as each new generation ignored his wishes and worshipped either Guthix or himself, he tried to make new species. This also failed. Ocellus soon went hunting for Runestones to help in his research, also visiting Juna soon after to collect the Tears of Guthix hoping to use them for further research. Though Juna denied him access to the tears, no longer agreeing with his plans, so he stole the tears from her instead. Using the tears with the runestones, he created crystals. He soon created the Legiones out of crystals, gifting them with more intelligence and better understanding. Still, even they failed to understand Guthix's wish. During the Assassination of Guthix, he was not called to go to Guthix's chamber with the other Guardians. After Guthix's death, he felt guilt, and blamed himself for not being able to help. Soon after, his own order went rogue, and started to experiment on people, hoping to find a way to create a new god of balance. Ocellus tried to confront them, but was defeated, so instead he chose to trap them, and guards the Monastery of Ascension so none can escape.
'Auriculus' is the diminutive form of ear (auris) in Latin, and 'Ocellus' the diminutive of eye (oculus).
Despite being a Guardian of Guthix, he is quite morally ambiguous; his journal reveals he attempted to breed humans eugenically for independence, neutering the worshipful, before destroying the progeny when the experiment didn't work out. He then started again and started selectively breeding for strength. He says that humans bred fierce wolves into docile dogs, and he wanted to breed humans in the opposite direction.
Ocellus looks strikingly similar to a blood reaver. This, coupled with his desire for control, his state of being a demon, and his usage of Latin, suggests an affiliation with Zarosianism. Blood reavers, a kind of virius, are loyal to Zaros.
The crossbows made by Ocellus give immunity from all factions in the God Wars Dungeon, much like Nex armour and weaponry. Repairing the crossbows via a smith stand also calls it "ancient equipment," further suggesting a Zarosian link.
Dialogue with Zaros during and after Fate of the Gods reveals that Ocellus, along with his brother Auriculus, were once Zaros's pets.