Most importantly, Ogu had dangled a Lord's Stone as bait, proposing a series of conditions that Torin simply couldn't refuse.
Gradually, Torin gave up resisting and chose to cooperate. He had no choice but to accept Ogu's plan: betray the human kingdom and corrupt other races to build an army to fight against it.
In his heart, Torin knew he was probably just a pawn, doing all the dirty work for Ogu's benefit. But to advance to the Legendary level, he had to agree.
If he continued down the normal path within the human kingdom, becoming a lord meant being shackled by the rigid nobles system. Even after paying an enormous price, his chances of obtaining a Lord's Stone were minuscule.
In short, Torin had figured it out. He had no future in the human kingdom. That was why he chose to work with Ogu.
