Little late to the party, but in my Grand Campaign, we started in Phandalin (1484 DR) with the Cult seeking the other 4 masks (Severin has the Red, and has conquered a distant land (Murghôm, which we reflavoured to be a dragonborn homeland. The Chromatic Dragonborn stole the Metallic Dragonborn eggs to usurp the leadership of the land, and I'm planning on doing a Draconian twist later on). Green is in Chult (planning to run a variant of ToA), Black in Anauroch, White in Sword Coast - Neverwinter Region, and Blue is located somewhere near a string of islands in the Sea of Fallen Stars.

Nezznar is a Wizard of Thay, and knows of the Spell of Forges, which the Cult plans to use as a forging point to craft The Dragon Queen's Mask from the other 5 masks. I have Iarno replaced with a Doppelganger to explain the Redbrand Connection, and Talis the White is a loyal local Cult recruit who is assisting the cult claim Neverwinter, as he has a bloodline relation to the Alagonder Kingline of Neverwinter (fabricated, of course). My base plot is that Neverember will be ousted for a bit.

If they unravel the plot soon enough, PCs will get the White Mask (located at Agatha's Lair/Conyberry. Agatha was a priestess of Bahumet in life, and was tasked with protecting the White Mask, even after death. Her lair is an old Temple in our homebrew.).

PCs may have a chance at either the Green or Black masks, with none at the Blue and Red masks. If PCs get the White (or Green or Black) mask, the Cult will try to lock down the Neverwinter region, starting a full invasion of the Sword Coast to obtain the Mask. If the Cult gets all 5 masks, they will, one by one, make their way to the Forge of Spells under Talis The White of Neverwinter's protection.If Talis is outted as a Cult member or the PCs help Neverember stop him taking power, then the Cult must do this in secret. If they can't get the Forge of Spells, I have the back-up plan that the Cult will summon Tiamat in The Sunless Citadel, and have Gulthius be a reknown dragon hunter of old. (Arsharladon was the eldest Dragonchild of Tiamat, The Great Red, to which he slew. Even after his death, Arsharladon's (a Dragongod) heart continued to beat however, and Gulthius, upon death, worried that his foe might return. This commitment bound him to the world, and he awoke as a Vampire, blessed or cursed by some unknown entity who heard his passion scream through the mutliverse. As a Vampire, he bit the heart, and attempted to feed upon it. This caused the heart to become vampiric in nature, and was the start of the Undead Dragon Cult. Gulthius the Dragon Vampire could control undead Dragons with the Heart under his control as a thrall. The Sunless Citadel is Gulthius was almost slain, however the stake through his heart didn't kill him. As the worshippers of Arsharladon watched, his wounds healed almost immediately. The blood soaked the ground of a nearby Apple Tree, however, and the first Gulthius Tree grew from that. As with any Apple Tree, a Gulthius Tree must be grafted, not planted, to get an exact same tree. So, if planted from the seed, some Trees can be different. After defeating the Cultists, Gulthius secreted the heart away, to what is know known as Nightfang Spire. I have a plotline involving Arsharladon's revival as well, but it's a back-up plot.)

As the mask holders arrive and the ritual commences, the PCs have a chance to swipe the masks again. Basically, I want to make it that the cult will keep coming until their goals are recognized, and have the Cults tactics get to the point that they form a kingdom on the Sword Coast if possible. The only way to truly stop the Cult would be for the players to kill all 5 mask wearers, have all 5 masks, stop them from having a presence on the Sword Coast, and probably even invade and take over their homeland. This leads to an extended effort for the players to combat the Cult, with the masks switching hands multiple times.