The Plunder Patriarch is a very rare treasure monster, that can drop a piece of the new best slot equipment that will be introduced. It is necessary to kill a particular primal beast with some sort of shock treatment to have the resonance altered by Gnomadness and its device, thus the "risk level" is increased. The higher the risk level, the greater the chance that the Plunder Patriarch will appear.
The patriarch has cooperative loot. It drops one of the BiS items (unique drop) per kill, in very rare cases it may drop two. Those who contributed damage to the kill have a chance to loot this item.
It's a rare spawn and as mentioned, the spawn chance increases if you hunt with a higher risk level. While there are similarities to Soul War, the scale is different.