Guild Wars 2

Based on the collection for Ad Ifinitum I wouldn’t surprised if it was a gift which you have to empower somewhere. Since Ad Infinitum collection required wasting a Gift of Metal and Wood, my guess would be nothing less than a Mystic Tribute. I mean armour is obviously a much bigger deal than a back item.
So yeah one last slap in the face before you are done at last.
I could be wrong of course, but since it’s only 1 item left it probably is something “special” rather than related to killing a raid boss again.

They already left a clue in their post “…To complete the collection, players will need to visit and interact with the only living thing remaining in the Bastion of the Penitent once the prison has been cleared….”. With this said I think the only living thing at the end of w4 is that strange tree near Deimos fight zone.

Zaraki.5784:

…I think the only living thing at the end of w4 is that strange tree near Deimos fight zone.

I believe you are correct.