Rescuing Rachana not only allows them to avoid the wrath of Lord Raven. As the best vessel made by the Spectre so far, she would soon contain and merge with Birsha's spirit should Lord Raven's earthly body finally die.
Although most of the people thought that the 'vessels' were actually people who had a fragment of spirit within them, the real vessels were actually Lord Raven's future bodies.
All the people who were called 'vessels' had the spirit fragment with them. That is because the Spectre was trying to ascertain and condition their bodies to become the spare bodies of Lord Raven.
Some of them would completely absorb the spirit fragment, while others would just remain stagnant, at most they could only manifest stronger curse power than curse users without a spirit fragment.