Prof.Dr.Satya Vrat Shastri's Thai Ramayana 36.Ravana's soul

Ravana, his efforts to conquer Rama without a battle having gone in vain, entered the battlefield. The battle so frightful was never seen or heard before in three worlds! Siddhas, yakshas, sages, serpents, gods and other creatures came to see the battle.

 Rama showered a volley of arrows on the lord of the demons, but Ravana surprisingly was not afflicted. A limb which was cut joined again. Vibhisana, the wise brother of Ravana said, 'The soul of Ravana is deposited in a cage and the cage is at the abode of his preceptor, the seer Goputra. Until this is killed it is not possible to slay him.'

Hanuman acts swiftly and seeks permission from Rama, 'I shall do this work and very soon your anxiety will be removed'. And adds 'If I resort to some deceitful course, you should never look at me with any doubt even in your thoughts.' Rama says, 'So be it.'

There is a long story of how Hanuman along with Angada meet Goputra pretending to change sides as he is not happy with Rama. Convinces Goputra to take him to Ravana and help him switch sides. As Goputra is worried about leaving the soul of Ravana behind, he carries it with him. As they reach Lanka, Goputra remembers that if they cross the border and get into Lanka, Ravana's soul will merge with his body!.
Again Hanuman convinces Goputra that Angada will take care of it till they return and Angada would wait at the border.  Goputra agrees!

A little while later Hanuman tells Goputra that he has to convey  something important to Angada and promises to return quickly. Then in a moment Hanuman meets Angada, takes out the soul from the cage and replaces it with a new soul he has created.  Then directs Angada to dump the original soul on the sea-shore in such a way it cannot come out. Ravana unable to connect with his soul meets his end by the sharp-pointed arrows showered by Rama.

The story concludes with very interesting observations:
There is no doubt that for the achievement of the object of their lord, servants, intent on it, stake even their lives.
'One should not trust an untrustworthy person. Even a trustworthy person should not be trusted.' This good story of Thai Ramayana confirms this. 
Goputra, very simple by nature, reposed his full confidence in the son of the wind. For that he came to be cheated by him.
Having trusted the enemy he became the cause of the destruction of his most majestic and the most dear pupil.

It seems the souls of Rakshasas are very different from that of humans.The belief that only human souls are immortal  is applied in this story.