Hoard of the Dragon Queen/Rise of Tiamat
The Cult of the Dragon has been active in Faerûn for centuries. It has focused on making undead dragons to fulfill a prophecy most of that time, but that’s changing.
The new leader of the cult is a Calishite named Severin Silrajin, who believes that real draconic knowledge and power belongs to living dragons, not undead ones. Severin’s ambition amused Tiamat, so she revealed the existence of five dragon masks to him—one for each chromatic dragon color. Individually, these ancient masks allow wearers to communicate with dragons. More importantly, a person who is erudite in draconic lore becomes a wyrmspeaker while wearing the mask, which allows the wearer to think like a dragon, gain favor among dragons, and subtly influence their behavior. When all five are brought together, they magically merge into a single Mask of the Dragon Queen. With the assembled mask, the cult can release Tiamat from her prison in the Nine Hells. After Severin (subtly guided by Tiamat) discovered that secret, he bent all the cult’s resources to finding the long-lost dragon masks in their secret hiding places. When he recovered the red mask, Severin became the first of the wyrmspeakers, but others soon followed.
As the Cult of the Dragon has grown bolder, its actions have drawn attention. Your characters have stumbled onto the Cult’s scheme in some manner or has a connection to dragons.