“Sinkers” is the apparently nickname given by denizens of The Elemental Plane of Fire to a group of individuals who attacked the party during the incident at Mount Nograd. It is a name for the group that the party has since taken on.
Despite the heat of the plane they were in, they were dressed heavily in armour, beneath the helms of which the only features of these people were visible, a cold glowing blue light where eyes would normally be. Their goal seemed to be to rip down the boundaries keeping the planes separate from eachother, and it was this goal they single-mindedly strived to achieve at the battle. Even despite this, these warriors showed great prowess in both fighting and magic, and were it not for them simply ignoring most of the party the battle may have swung in a different direction. Instead they focussed on disrupting the closing ritual of the rift between the planes, a feat that was accomplished to some extent.
A second attack was experienced where they fought against defenders in Arborea. Once again they are heavily armoured with glowing blue eyes. However once dead, removal of the helms revealed a variety of different races, even some extraplanar ones. There was an apparent leader in this battle, who has also been the only Sinker to show any sign of a marking, wearing an amulet around its neck.
Upon entering the Underdark to investigate the Lich, the party eventually found him in companion with another one of these Sinkers, one that also had an amulet. In the ensuing battle, the party focussed in breaking the amulet so the leader could not disappear as the previous one had. This proved successful, and allowed them to capture the Sinker leader. The leader, however, turned out to be arrogant, annoying and cocky, and wasn’t much use in interrogation. Eventually he was put to death and buried. The only clue the party now had was the, now partially broken, amulet. For safety and expense, they decided not to repair it.
Following some leads on the Yuan-ti eventually lead the party to a single-standing tower, seemingly just plonked down in the desert, apparently some sort of Sinker holdfast used to drive the Yuan-Ti out of their temple. The party found abject chaos inside, with almost no sign of human life, although plenty of the tower was alive, including the tower itself. Upon rescuing a Sinker from this madness, they discovered that two portals were opened here, one to the Mechanus through with many of them fled, and one to The Positive Energy Plane, which would explain the live tower now rolling around the desert somewhere. The Sinker seemed to know little, and appeared to have joined the Sinkers voluntarily, but with a different goal in mind. She was let free by the party.
The party eventually tracked down some other Sinker refugees in the Mechanus, discovering them to be dying, and frantically trying to look for answers as to why. It seems that each of them had joined for a different reason, being promised different things, and all had almost no memory of what occurred from the moment they joined to some recent event, which the party put down to them sundering the amulet. With the amulet only partially destroyed and their general killed, the Sinkers under its control started having adverse reactions to the energy within them, causing it to slowly kill them from within. Without much other choice, they accepted a suggestion from Kerys Nae Gwennen to travel to the base of The Spire in The Outlands, where magic ceases to exist.
Only two of the Sinkers made it, the other expunging as much as he knew to the party upon his deathbed, stating that there were 3 other contingents of Sinkers led by generals. One of whom loved to be on the battlefield with the troops (presumably the one they encountered in Arborea beforehand), one that remained holed up in their stronghold, and one that acted entirely unseen. This seemed to be contrary to previous info about them all having strongholds, until it was theorised that stronghold can be interpreted in many ways. The sinker also sputtered something about dreams, and the amulets, and some planar scrying devices. The surviving two sinkers are holed up at the base of The Spire until the party has found a solution.