Ravan went to his Marich and asked him to assume the form of a golden deer and tempt Sita. Marich advised Ravan against doing so and also reminded Ravan that though the latter could not be vanquished by a Rakshas, god or Gandharv, he was susceptible to the arrows of a man.
Ravan ignored his advice, the plan was that when Ram tried to capture the golden deer for her, Marich was to run away and make Ram and Lakshman follow him, so that Sita would be alone, and Ravan could abduct her.
Accordingly, Marich assumed the form of a golden deer, and went to the hermitage, where Ram, Lakshman and Sita were residing. On seeing the beautiful deer, Sita asked Ram to get him for her.
Lord Ram, who very well knew that there was nothing like Golden deer, yet left Lakshman behind, and followed the deer, which took him far into the forest, and then he shot it.
Marich immediately assumed his own form, and then cried out for Lakshman and Sita in Ram's voice and died. The cry was meant to mislead Lakshman, who, thinking that it was Ram, would run to help, thus leaving Sita alone. In this way, with the help of Marich, Ravan succeeded in kidnapping Sita.