The change of aura originated from his body’s new power. This was Garen’s biggest achievement. If it were before he surpassed his totem light, the blood-colored aura looked strong, but in fact, couldn't be directly used on totem users. Its capabilities were to be a supplement to corrosive poison to corrode the other party’s totem light, and it was a large, area of effect ability. And now, it had returned to its original purpose.
Other than the aura, the other thing was the Cthulhu’s Origin.
The Cthulhu’s Origin was a treasure that allowed the Obscuro Society's marshalls to reach peak fifth form. It's effects were definitely supernatural.
However, to use the Cthulhu's Origin, one has to remove the Cthulhu King’s will. To do this, one has to first use Hannet’s purification abilities to draw itn out, then destroy it with an external force. Only then could the Cthulhu’s Origin be used properly.