The tracks
Mhysa and
Dark Wings Dark Words sound very similar, the first is clearly Dany's theme, the second track has the same melody, but is darker and gradually changes into a somewhat hopeful rendition the main theme.
Mhysa obviously is Yunkai releasing the slaves and them shouting 'Mother, mother' to Dany, it's very solemn and sacral, fitting the saviour imagery of the scene. I assume they'll cut from that to Catelyn's resurrection as the final scene before the credits with 'Dark wings, dark words' as the score. It fits the traditional supernatural 'holy shit' endings of the first two seasons.
Combining these two scenes with that music will make for a very emotional, uplifting ending compared to the last episode and it gives the title 'Mhysa' a double meaning, which they like to use for the episode titles. It will be epic!