This increases the chance of a reinforcement occuring.

You should go to this Leyline and start running towards the bridge you originally went across to unlock this Leyline. There will be a horde of Kaiwan demons that spawn near the middle of the bridge.

Wait until enough Kaiwan demons spawn and initiate a battle with one of them.

Doing this consistently in this area and other areas you find work can be a great way to grind without getting the DLC.