It was...
Great and Small Twelve Heavens Secret Demon Technique: The Supreme Secret Technique of the Demon Sect, the ultimate attack skill. Once used, it envelops an entire realm, gradually transforming all the spiritual energy within the realm into demonic Qi and forming a prohibition. Once the transformation is complete, the demonic Qi could be turned into Demon Thunder. On activation, chaos would reign between heaven and earth, reducing all beings to dust, leaving behind only a dead zone!
The Secret Demon Method, used once more!
Although the spiritual energy of heaven and earth had been seized by the ritual, making it difficult to transform into demonic Qi, Xu Yang had the Asura Seal, a third Order Demon Spirit Treasure. He could create the Nether River Blood Sea and use the Asura Demon Soldiers as offerings to extract the Demon Soldier Cloud Power and condense the Secret Demon Divine Thunder. It was possible to execute this Forbidden Technique, albeit with reduced power.