In life Irvin was an unsuspecting buck. He sought after ways to improve himself, eventually turning to magic to contact the king of the underworld himself in hopes of achieving his goals. Hearing Irvin's pathetic request, Asathanos obliged. Stealing Irvin from the mortal plane, Asathanos brought him to the underworld where he reigned and transformed him into a powerful beast that would serve him until the end of time.
While not exactly what Irvin desired, he finds joy in his other form.
Irvin spends most of his time around Asathanos' estate, tending to the menial duties that the lord should be overseeing. Irvin is free to roam about the underworld as he pleases but cannot resist when Asathanos turns him into his monstrous form. Asathanos occasionally does this solely to annoy him. Regrettably he is unaware that Asathanos is only using his monstrous form to further strengthen his forces against the inevitable ambush to overthrow Adontis.
The buck often finds his work stressful as Asathanos prefers to indulge in lavish parties instead of complying with Irvin's nudges to get his work done. Silently Irvin envies those that get to be intimately close with the bull and wishes Asathanos would see him as more than a mere servant. Unfortunately for Irvin, he finds himself incapable of admitting his feelings for the bull and instead flounders in flustered turmoil whenever Asathanos gets too close.