The story takes place in the darkest moment of seventh grade in the Harry Potter books. The trio were on the run at that time, and Snape, who had become the headmaster at this time, received a mysterious letter.
This is an ordinary letter, but the fault lies in the dazzling green and ink handwriting on the envelope, the slightly raised and unique "g", and the affectionate title that I haven't seen in a long time - Sif
"Dear Sif, long time no see. I wonder if you still remember me, the little red-haired girl who entered the magical world with you when you were a child? I wonder how you were doing there..."
While Snape was staring at the letter, Dumbledore on the wall actually showed a "weird" smile.
"Severus, don't you want to know who the owner of this letter is? Don't you want to know how she sent it to you?"
"So, this... This is really from her... Is she not dead... But I clearly saw it... I clearly saw it with my own eyes... This is impossible..."
"Yes, you are right, she is indeed dead..." Dumbledore said seriously, a sharp light flashed in his blue eyes.
"No one can transcend life and death..."
Concept: Snape's self-salvation and Lily's two-way journey