Complex sentence: Elena and many others are able to pass back to the land of the living afterwards, but the spell that allows this is broken before Damon passes back through, and Bonnie is stuck as the anchor, leaving those alive mourning the loss of Damon and Bonnie.
Split into simpler sentences: Elena and many others are able to pass back to the land of the living afterwards. But the spell that allows this is broken before Damon passes back through, and Bonnie is stuck as the anchor, leaving the gang mourning the loss of Damon and Bonnie.