Elder Scrolls Online (ESO) Blackwood deals with the fallout from a daedric deal made between the Longhouse Emperors and Mehrunes Dagon.
There were three emperors in this dynasty—starting in 2E 533 and ending with the Colovian Rebellion in 2E 577 led by Duke Varen Aquilarios of Chorrol. These events take place in the years immediately preceding this installment in the franchise. Now, it appears that the Vestige is being called back to uncover what really happened to the few Reachmen who held the Ruby Throne.
The "Longhouse" part of their name comes from the type of structures the Reachmen would construct and live in.
Elder Scrolls Online: The Longhouse Emperors
The Longhouse Emperors were a short-lived dynasty of Reachmen leaders who took the Ruby Throne by right of conquest through the help of the daedra—specifically the Daedric Prince of Destruction, Mehrunes Dagon.
The first of them was Durcorach the Black Drake. According to commissioned manuscripts of his life, Durcorach was an incredibly strong man who performed feats difficult for adult warriors at the age of seven. He eventually became chieftain of his individual clan and had a notably large following in 2E 527. This caught the attention of the Tagh Droiloch witch coven, who helped him completely conquer the reach through daedric magic and deals. By the end of 2E 529, he had amassed an army of over 10,000 strong.
Durcorach and the Tagh Droiloch eventually struck a deal with Mehrunes Dagon to have the Reachman warlord crowned Emperor of Cyrodiil—which came to fruition in 2E 533. For their part of the deal, the Reachmen were to await the day when Dagon would come to collect with the Four Ambitions.
As Emperor, Durcorach wed Veraxia Tharn to legitimize his claim and led several invasions into High Rock. He captured Hallin's Stand and eventually sacked Evermore in 2E 542 before turning his armies to Wayrest and, finally, Daggerfall. Unfortunately for him, this action would ultimately be his downfall, as Emeric of Cumberland led an assault to flank the group and protect the city. Durcorach died at the hands of Emeric and his armies dispersed.
Moricar the Middling, son of Durcorach, took over after his father died. He continued his work with the Four Ambitions and obediently awaited the reappearance of Mehrunes Dagon. To this end, Moricar built a structure beneath the Imperial City to house the Ambitions. Otherwise, aside from a brief failed invasion of Western Skyrim, Moricar's rule was uneventful. He was succeeded by his son, Leovic, in 2E 561.
Leovic was the last of the Longhouse Emperors, reigning from 2E 561-577. He married Clivia Tharn and commissioned the first edition of The Emperor's Guide to Tamriel. Leovic wanted to show Tamriel the culture of the Reach and, to that end, moved to legalize daedra worship in 2E 567.
This angered several citizens who rebelled against him and his rule. The Colovian Rebellion, as it was called, was led by Duke Varen Aquilarios of Chorrol and sought to reclaim the throne from the Longhouse dynasty. Leovic attempted to reach out to Mehrunes Dagon during the war but was ultimately met with silence. He ordered the Four Ambitions to be hidden away before being struck down by Aquilarios in the throne room.
Only Leovic and his steward, Farrul Lupus, High Priest of the Order of the Waking Flame, knew where the secrets of the Four Ambitions were hidden.