Four Is The Best.

GREED just can't trust the realm lord, knowing the things it knows. First, the realm lord made a show of uniting the abyss with the realm through the abyssal coalition. It was not a mere show for others to see. It was a believable show that also had a useful purpose. 

The demons of the abyss couldn't betray the realm after the oath, and they are doing well in the Tyrant Realm, helping the High Heaven realm win this era of conquest. Talk about killing two birds with one stone. But the realm lord didn't end there. 

The realm lord used the demons to create a traitor that can betray the realm as a trap for his enemies to fall into. That makes the third bird killed with just one stone throw.