Shantih shantih shantih
We can see the coexisting themes of incompletion and unity in the repetition of this last stanza, which is an extremely dense reflection of the entire poem. It begins with the Fisher King’s illness as evidenced by the “arid plain;” then, we are taken back to the beginning of TWL when the London Bridge falls down, perhaps foreshadowing “a heap of broken images” (line 22). “Falling down” is repeated three times, echoing the recurring theme of the trio that we find throughout TWL. The “swallow” referencing Philomela is only repeated twice, hinting at incompleteness. The line referencing the Spanish Tragedy inserts the theme of revenge, also a loose end. Yet the stanza ends with “Datta. Dayadhvam. Damyata,” a saying that unifies the paradoxical natures of gods, men, and devils, and finally “Shantih,” inner peace that transcends human understanding.
I think that this incompleteness (such that we see in the story of Philomela) was intentionally left unresolved in contrast with the unity and inner peace that concludes the poem. In a very broad context, I think Eliot means to convey that humans can find this sense of completeness in spite of, and perhaps even because of, the loose ends that characterize human life.