In Goswami Tulasidas's Shree Ramcharitmanas, Bhagwan Shiva tells Mata Parvati that the Supreme God takes the avatar and descends to Earth for various reasons. One cannot pinpoint and say that Bhagwan takes avatar "only for this" reason.
सुनु गिरिजा हरिचरित सुहाए ।
बिपुल बिसद निगमागम गाए ।।
हरि अवतार हेतु जेहि होई ।
इदमित्थं कहि जाइ न सोई ।।
sunu girijā haricrita suhāe
bipula bisada nigamāgama gāe
Hari avatāra hetu jehi hoi
idam ittham kahi jā'i na soi
Meaning: "O Parvati! Listen, the Vedas and scriptures have sung the beautiful, detailed, and pure characters of Shree Hari. The reason for which Hari takes avatar cannot be said as 'this is it' because there can be many reasons which one's material intellect cannot even comprehend."
The fundamental reason for Bhagwan's avatar is to establish dharma by defeating Rakshas, spreading God's glory, and giving bliss to the devotees. Bhagwan Shiva then narrates different avatar stories of Bhagwan Ram and Ravan in various kalpas. Here, we list those stories.
The Cycle of Curse and Redemption: Jay and Vijay's Journey from Vaikuntha to Earth
The Sanat Kumaras go to Vaikuntha to get darshan of Bhagwan Vishnu. However, two of Bhagwan Vishnu's guards, Jay and Vijay, block their path. A misunderstanding ensues, and ultimately, the Kumaras curse Jay and Vijay to be born as Rakshas on Earth for three lifetimes. The curse sees them take the forms of Hiranyaksha and Hiranyakashipu, creating chaos and destruction during their time on Earth. To restore peace, Bhagwan Vishnu takes on the avatars of Varah and Narsimha to defeat them. However, Jay and Vijay are not done yet; they return to the mortal world as Ravan and Kumbhakarna. This time, Bhagwan Ram takes the avatar and defeats the duo.
The Tale of Sage Narad's Curse and the Birth of Ravan and Kumbhakarna
Naradji goes to the Himalayas to perform Tapasya. As usual, Indra assumes that Narad is doing Tapasya to claim his throne, so he sends Kamdev to break his penance. However, Narad's Tapasya is so strong that Kamdev fails to break it. Narad's ego inflates after defeating Kamdev, and he goes around boasting about it to Bhagwan Shiva and Bhagwan Vishnu. To teach Narad a lesson about pride, Bhagwan Vishnu creates a grandiose town where the king, Sheelnidhi, organises a Swayamvar for his beautiful daughter, Vishwa Mohini. Narad falls in love with her at first sight but lacks confidence in his appearance. He goes to Bhagwan Vishnu and asks him to make him as beautiful as Hari. Instead of giving him a beautiful face, Bhagwan Vishnu gives him a monkey's face. Narad is unaware of this and attends the Swayamvar, where everyone mocks him, including two Shiva Ganas disguised as Brahmins. They tease Narad the most, and as a result, Narad curses them to be born as demons. Naradji is so humiliated that he curses Bhagwan Vishnu also to bear the pang of separation from his beloved wife. To fulfil Narad's curse, Bhagwan Vishnu takes the avatar of Bhagwan Ram, and the two Shiva Ganas become Ravan and Kumbhakarna.
Jalandhar's Rebirth as Ravan
In the past, Kalpa, a demon Jalandhar, wreaked havoc in the celestial realms, leading to his defeat by Bhagwan Shiva. Jalandhar is reborn as Ravan, the mighty Rakshas king of Lanka. Ravan's strength and arrogance from his previous life carry over, leading him into conflict with Bhagwan Ram. Bhagwan Ram, in his divine form, fights Ravan to restore dharma. The battle symbolises the triumph of righteousness over unrighteousness.
The Reincarnation of King Pratapbhanu and the Birth of Ravan
A virtuous King named Pratapbhanu, with the help of his younger brother Ripumardan and the minister Dhramruchi, defeats all his enemy kings and performs thousands of yajnas. One day, while chasing a wild boar, he encounters a supposed sage, who is actually one of Pratapbhanu's enemies. The unsuspecting king is so impressed that he agrees to a devious plan suggested by the sage. The imposter proposes that Pratapbhanu organise a year-long feast for Brahmins in his palace. This feast would satisfy the Brahmins, and Pratapbhanu would become the undefeated king of the entire planet for thousands of years. However, with the help of a demon named Kalketu, the imposter hatches a plot to trick the Brahmins into eating impure food and makes Pratapbhanu responsible for it, leading them to curse him.
Unfortunately, the plot succeeds, and the Brahmins curse the innocent King Pratapbhanu and his entire family and ministers to be reborn as demons. As a result, the king becomes Ravan, his younger brother becomes Kumbhakarna, and the minister Dhramruchi becomes Vibhishan. Bhagwan Ram defeats them to rescue them from their demonic life.
The Divine Boon to Manu and Satrupa and Reincarnation as Bhagwan Ram's Parents
Manu and Satrupa, a devoted couple, undergo Tapasya (penance) with the desire to have a glimpse of the Supreme Bhagwan in His Ram form. After years of dedicated penance, Bhagwan Ram and Mother Sita manifest before them and grant them a boon. Manu and Satrupa wish for a son who would resemble Bhagwan Ram. The Bhagwan replies that no one in the universe is precisely like Him, but He will take birth as their son and bless them with the honour and happiness of being the parents of the Supreme God. Therefore, Manu and Satrupa are reborn as King Dashrath and Kaushalya, respectively, and Bhagwan Ram takes birth as their son during the Tretayug.
देखि प्रीति सुनि बचन अमोले ।
एवमस्तु करुनानिधि बोले ।।
आपु सरिस खोजौं कहँ जाई ।
नृप तव तनय होब मैं आई ।।
dekhi prīti suni bacan amole
evamastu karunānidhi bole
āpu sarisa khojāu kaham jā'ī
nṛpa tava tanaya hoba mai ā'ī
Meaning: "Seeing the king's love and hearing his invaluable words, the Karunanidhi Bhagavan said – so be it. O, king! Where can I go to find [another] like myself? Therefore I, Myself, will come and become your son."