The reason for Tang Yin's headache was the Heaven Devouring Abyss. Or to be more accurate, the resources he would have to accumulate to replenish the Heaven Devouring Abyss each time he condensed a spirit vein.
Tang Yin could replenish the Heaven Devouring Abyss one more time by devouring the remaining corpses of the Demonic Beasts. However, this would only allow him to condense one more spirit vein. He would have to find another source to replenish the Heaven Devouring Abyss again.
Not only this, the amount of essence of the Heaven Devouring Abyss each spirit vein needed to be condensed increased each and every time. This only increased the quantity of the resources Tang Yin would have to gather to replenish the Heaven Devouring Abyss in the future.
Tang Yin did not think of this at the moment though, he wanted to condense his third spirit vein before thinking of the future. He left the cave abode.
Please don't forget to give me your powerstones! Would be a huge help to me, and will motivate me to continue writing more.