For years, people have been receiving mysterious fragments or envelopes. These people are strangers, nothing connecting them, they don’t know about each other.
Nobody knows where the envelopes/letters come from, who is sending them, or what the contents mean. Slowly but surely, a pattern appears. The recipients start to find each other, they start a little community of “victims” of the envelopes, and then one of the letters does something magical or strange. Something that suggests all envelopes are actually magical and there’s a deeper (terrifying?) purpose behind them.
As the story unfolds, you follow the recipients as the envelopes take over their lives, as they discover the puzzles inside and try to solve them, as they barrel towards something bigger. (Maybe it’s a test trying to see who can figure it out/will persevere?)
Could this be Romantasy?