Ramayana - Ravana as Rama's Purohit

Ravana was a brahmin and the king of Lanka. He is also considered as a great devotee of Lord Shiva.

It's said that his race from maternal side was Rakshasa and Prajapati (descendants of Brahma) from paternal side. He was the greatest scholar and most learned person of his time.

In this context, also consider the fact that he was also the brahmin from Raja Dashrath's (Rama's father) side.

Now, when Lord Rama crossed the ocean and reached Lanka to battle with Ravana, he is supposed to fix the auspicious date and time of the war (muhurth).

Ravana being the only brahmin available nearby (that too purohit from his father's side), Rama sends Hanuman to Ravana for this.

Even though, Ravana knows that if fixes the date and time, he would definately be loosing the war, he did not turned back on this as it would have been against the brahmin rules.
And hence, It is Ravana who fixed the muhurth in favour of Rama and ...(we know the result)