After the destruction of the dark brotherhood within cyrodill, the imperial legions discovered the secret passage that led from cyrodill to the main HQ of the Morag Tong in Morrowind. With such access, the legions were able to eradicate 2/3 of the Morag Tong, uncluding their leader. The assassins were forced to fall back into Skyrim starting a new reign of terror and chaos.
Oh i feel poetic today.
Im not very good at creating quests in CK, but the wiki is great at giving tutorials.
Obeying the old law, a new leader was elected, and under the leadership of the dark elf, Daris Sevilo, the organisation underwent a major reformation including the change of techniques and the recruitment of new members.
Under these circumstances we discover that the people who desacrated the night mother's tomb in Bravil were not the imperial legion, but the Morag Tong, who have retrieved the blood of Matthiew Bellamont.
With the 'blood of the traitor', the organisation has used the old robe of Martin Septim to form a divine barier around the mundus, preventing the interference of the Sithis and banishing all members of the black hand who were specially bound to Sithis. The same way the dragonfires prevented opening stable portals from oblivion, now the barrier of Morag Tong prevents any access to the void.
And now the story beings, the assassins of the Dark Brotherhood, are on their own in the chaos that has disrupted.
But the foe's barrier is not as great as they thought, instead of only blocking Sithis, it turns out that the souls are actually coming back from the void, forcing graveyards and caves to become rally points for undead such as Rufio (Oh you remember him, oblivion players), Astrid or even Lucien Lachance himself.
Obviously when we start the quest, the power to summon the spectral assassin will be unavailable.