Cora hadn't meant to find the mirror.
Turns out in the wonderful land of the Underworld, there was a mirror that let the holder see into the Land of the Living. She quickly took to glancing through it every so often, to check up on her daughters and whatever they were doing.
It caused a few awkward moments for her that left Cora very nearly blushing and returning to her paperwork—seriously, what did her daughters see in Robin Hood? But one of her favorite discoveries was seeing Hook.
Hook hadn't just allowed the Swan girl to best him more than once and then toss her aside like Cora imagined him doing to other women. He'd fallen in love with her, completely and truly. Cora, after checking on Regina's messed-up life and Zelena's frankly disturbing life and wincing in sympathy and revulsion respectively, liked to think about Hook for a few moments and see what he was up to.
As time went on, he spent more of his life with Emma, working with her, adventuring with her, encouraging her, kissing her, and going on lunch dates with her. Cora can't help laughing at how far both of them have come; she's a changed woman with her heart and proper emotions returned; she can appreciate a good redemption story, unless it's her own.
Cora was busy with an uprising of assorted unresolved problems of people and didn't get the chance to look through the mirror for nearly two weeks. When she finally collapsed in her chair in the facsimile of Regina's office, she grabbed for the mirror.
Regina and Zelena and the others were in a different realm, probably Camelot. Emma was clothed in white but Cora easily saw the darkness swirling around within.
Over the next week, Cora watched diligently over her little band of Storybrooke people in Camelot. She saw the love between Hook and Emma more clearly with each passing day. Emma was trying to stay away from the darkness, and Hook was encouraging her in every way he could.
It touched Cora's heart, really.
The final day in Camelot was a blur. Hook's wound breaking open, Hook dying, Emma crying, Emma turning him into the Dark One, the Dark One killing Merlin, the Dark Curse. Cora watched the day unfurl with widened eyes, no longer focusing on her daughters, but on the two most evil creatures in the universe.
There was another uprising—the Underworld was growing restless—while the Dark Swan wreaked havoc in Storybrooke.
Cora watched through the mirror on the last day Hook was alive. She watched as the Dark One almost destroyed the family he loved. She watched as he became Killian again and forced Emma to kill him. She watched as Emma cried on the body of the man she loved.
Then she heard it. The faint clang of the dong that means someone arrived in the Underworld. It was Hook; she knew it.
But as Cora watched the mirror—she knew that Emma Swan would be visiting soon enough, to reclaim the man she loved.
