Nagini immediately knew that the image in the mirror wasn't real. Perhaps it was a trick, or perhaps it was a blessing, designed to provide sanctuary for the utter misery that was the real world. She didn't care which it was. After recent events, all she wanted was a little escape from reality.
It was night time in the mirror. The pale silver moon drifted lazily in a sky scattered with stars. Nagini's reflection, still very much human, was dancing and twirling across the grass, dressed in a lovely collared blouse and a simple skirt. She looked comfortable in this outfit - it seemed to offer so much more freedom that those horrible circus costumes.
The grassy landscape built up to a gentle hill in the background, and the whole ground was covered in shimmering, glowing dandelions. As Nagini watched, her reflection picked up a handful of dandelions and blew them, sending the glowing seeds soaring into the night sky. A bunch of fireflies passed by, flashing their lights as they weaved through the floating dandelion seeds. Nagini gazed longingly through the frosty glass, watching the symphony of shimmering, glowing lights.
From behind the grassy knoll, a second figure appeared.
Credence looked different. There was a confidence about him, a carefree spirit that Nagini had never seen before. He came flying down the hill with a huge smile on his face. As he neared the bottom, he seemed to trip over something and went tumbling down, falling gently onto the bed of glowing dandelions. Laughing, he accepted mirror-Nagini's proffered hand and clambered to his feet. But he didn't let go of her hand. He pulled her away, still laughing, up the hill.
The two of them got smaller and smaller until they became a silhouette at the peak, their hands still joined. Then, they disappeared beyond the hill, together running towards whatever was on the other side.