Demonic possession is a great taboo for cultivators. Unlike members of the Demon Race who absorb Demon Spirit Qi for cultivation, people who fall into demonic possession usually suffer a great blow to their equanimity, allowing the heart demon to invade. In plain terms, it is the burning of one's Origin Power, which unleashes strength far beyond one's current Cultivation Realm in a short period of time.
But the ultimate outcome is dire, at best resulting in a complete loss of sanity, plunging into the Demonic Path of slaughter without hope of redemption; at worst leading to death and extinction, with one's lifelong cultivation vanishing like water downstream.
Sun Wukong had already reached the Quasi-Saint Realm of Slaying Two Corpses. He should have experienced the tribulation of heart demons long ago, making his current fall into demonic possession an incredibly inconceivable event in the eyes of Tathagata Buddha.
"Emotional tribulation!"
