“Stay, for the wondrous deer we see The fiend Márícha’s self may be. Ere now have kings who sought this place To take their pastime in the chase, Met from his wicked art defeat, And fallen slain by like deceit.

In plain words: Lakshmana warns that the deer may be the demon Maricha in disguise. He says kings who came hunting here before were tricked and killed by such tricks.