Aura of Death 

When surrounded by a 'blanket' of black mist, Minos immediately began to feel that thing trying to penetrate his body through his pores.

Simultaneously, every fiber of his being was brutally attacked by that thing while he felt the sensation of having a predator trying to hunt him down.

But Minos' willpower was strong. He pressed his teeth together, trying to focus all his concentration on using that thing to purify his killing intent.

That would obviously subtly change his body, so he was undergoing a tortuous process in which the Aura of Death would try to improve specific attributes.

That was not explicitly speaking a marrow cleansing but an attempt to give a more capable body to withstand an additional soul ability.

Killing intent could not be considered a technique, much less a Physique skill. Instead, it was a passive skill variant, something that was called a soul skill.