Reason's of Raam Avataar? (3) - 
Jalandhar Raakshas

According to Maanas

In one Kalp there was a Raakshas named Jalandhar. His wife Vrindaa was a Pativrataa woman. Now this Raakshas was so powerful that even Shankar Jee was not able to kill him. Its another reason was his Pativrataa wife also. Her Paativrat used to guard him all the time.

So Vishnu had to break her Paativrat Dharm first, and then kill him. When his wife came to know about this she gave Shaap to Vishnu. Vishnu obeyed that Shaap. Jalandhar was born as Raavan and then Raam uplifted him by killing him Himself.

According to Bhaagvat Puraan

Its another story goes like this: When Vishnu had broken Vrindaa's  Paativrat Dharm, and she came to know about this, she gave Him Shaap - "You be a stone". Vishnu said - "So be it" and He became a black stone called Shaaligraam. Then Vishnu said - "You gave me Shaap and I accepted it, now I give you a Var, you accept it. You will be born as Tulasee plant which will grow in abundance near Mathuraa (at that time Madhu Van), and that Van will be called Vrindaa Van. I will also come there and live there for some time and I will not take anything without you." Vrindaa fell on Vishnu's feet and prayed Him.

That place was then filled with Tulasee plants and became famous as Vrindaa Van. Since then Bhagvaan Vishnu doesn't accept His Prasaad without Tulasee leaf.