Inside Chris' training cave, Jack was still working on fusing with the Runestone of the Devil King. He spent his effort all day long and every day just for the runestone. Outside the cave where Jack was hiding, Spark and Therras stopped the monsters from disturbing him. They followed his instructions and did not kill the monsters too fast. This way, the monster's spawning rate didn't increase.
Like the other runestones, there was a time limit to the fusion trial. In the other runestones, one passed the tribulation if they could endure the time limit without pressing the quit button. In the case of Runestone of the Devil King, if one failed to connect all the lights within the time limit, one was sent out of the tribulation space, and the fusion was considered a failure. One had to start from the beginning again.