Ravana showed Sita Rama's severed head to deceive her.

Ravana, the demon king of Lanka, was deeply infatuated with Sita, the wife of Lord Rama. In his relentless pursuit to possess her, he hatched a plan to deceive her and make her believe that Rama had been killed. Ravana knew that if Sita believed Rama was dead, she would be more likely to accept him as her husband.

To execute his plan, Ravana disguised himself as a mendicant and approached Sita in the Ashoka grove where she was held captive. He began to weep and wail, pretending to be a messenger sent by Rama's allies. Ravana convincingly told Sita that Rama had been slain in battle and displayed Rama's severed head as proof.

Impersonating a mendicant and pretending to be a messenger:
Ravana's disguise as a mendicant allowed him to approach Sita without raising suspicion. By pretending to be a messenger, he gained Sita's attention and trust, making it easier for him to deceive her.

Feigning sorrow and presenting false evidence:
Ravana wept and wailed, convincingly portraying a sorrowful messenger delivering tragic news. This emotional display was intended to manipulate Sita's emotions and make her more vulnerable to his deception. To further solidify his claim, Ravana showed Sita Rama's severed head, which he had conjured through his mystical powers.

Psychological impact on Sita:
Seeing what she believed to be her beloved husband's severed head was an incredibly traumatic and distressing experience for Sita. The sight of Rama's head would have shattered her hopes and filled her with grief and despair. Ravana aimed to exploit Sita's emotional vulnerability, making her more likely to accept him as her husband in her state of distress.

In conclusion, Ravana showed Sita Rama's severed head to deceive her. His impersonation as a mendicant and messenger, combined with his emotional manipulation and false evidence, created a convincing illusion of Rama's death. This was a crucial step in Ravana's plan to deceive Sita and make her believe that Rama was no more, ultimately aiming to coerce her into accepting him as her husband.