I heard about these guys in Oblivion based on the character http://elderscrolls.wikia.com/wiki/Greywyn_Blenwyth with the Vile Lair.

I think it would have been cool to join a vampire coven or vampire faction like this in Oblivion. They had several vampire factions you could join in Morrowind and in the Dawngaurd dlc with Skyrim. These guys seemed really interesting how they served Sithis and the Black Hand in their own way. svcks they were all killed though. Seems most vampire covens made easy targets in Cyrodil or just warranted vampires a death warrant for being in one.

Why does Sithis hate vampires anyway?

There is nothing that indicates that Sithis can or does "hate" at all since "he" is absent of emotion so he has no inclination towards or against Vampires. Also since every sanctuary we have ever seen in the Dark Brotherhood has had at least 1 Vampire that would indicate that their fates tend to favorable in the service of the Dread Father

For the record, please use UESP instead of the ES Wiki - it is much more reliable (heck, even the first sentence of the link article contains false information - we have no clues whatsoever about Blenwyth's race).

They didn't really serve the Black Hand - the Scars were Greywyn's way of trying to take over the DB and make it a vampire-only faction. Which is why they were purged.

Wait? Sithis hates vampires?

There is like a vampire DB member in every game. I would figure he liked them, if Sithis even has emotion, I haven't really found anything that indicates such.

My mistake about Sithis hating vampires.

I now realize that this was the case instead.

Still it would have been a cool group to join, despite the fact Greywyn pissed off the Dark Brotherhood when he made the scars.

So, was the idea of the Crimson Scars to assist the Dark Brotherhood or how was this supposed to work anyway?

No, the original idea was to have other vampires join the DB (at the same time turning some non-vampires into undead), kill anyone who wouldn't be one and take over the position of Sithis-worshipping assassing group feared across Tamriel.

Come to think of it, wasn't that destroying the Dark Brotherhood in a way by turning them into a coven of vampires?

That depends on what you consider the Dark Brotherhood being. If you see them as a bunch of crazy murderers, clinging to worship of a being that cannot be what they consider it is, than nothing really changes. If you see something more, it is a possible view of what would happen.

The Crimson Scar sought to replace the DB however their only failing would have been that all Assassins would share weaknesses and even though Vampires are stronger than mortals as far as we could tell they would have only enlisted Cyrodiil Vampyrum (the Order) into their ranks. This means that they would have been limited since they were only ever present in Cyrodiil whereas the DB spans all of Tamriel (other than Morrowind)

I don't think Mephala cares if her/his assassins are vampires or not, they just love causing problems between two groups.

Ive heard that theory before...it is obviously crap but ive heard it...

More sense then a primordial force being in charge xD it's like making a death cult centered around gravity.

In Skyrim Lucien Lachance returns from the Void, still faithfully speaking of Sithis and his existence. Besides we already know that Sithis is Padome (or whatever the spelling) the force of pure chaos in existence. Besides Sithis never "does" anything for the guild, like miracles or intervention of which a Daedra would actively do if they wanted to

Lucien Lachance could be lying or could see Mephala as sithis, getting the wool pulled over his eyes. Padome is a primordial force, he has no true conscious anymore nor does he have any use for souls. Mephala on the other hand has use for souls and loves it when mortals kill using cunning and skill. Also in case you didn't know this but not all Daedric princes actively talk to their followers or give any sort of intervention or miracles.

There would be absolutely no reason to deceive someone after death...none whatsoever so if it was Mephala she would have told him the truth and laughed about it. Also there is no reason for him to lie...spirits have no reason to lie because they cannot have goals or ambitions, no reason to alter anything. Just because Sithis doesnt need souls doesnt mean that the Dark Brotherhood cannot worship him and try to send him souls. That is like saying that because a friend doesnt need money that it is impossible for me to throw money at him. They dont worship Mephala, they worship a known entity and since it IS a known force the kills do not praise mephala

The Dark Brotherhood branch of Daggerfall certainly worship Mephala.

And of course 1 branch out of the absurdly high number (before the great war) represent all of them right?

The Daggerfall branch covered the entire Illiac Bay region, so it was a great number of Sanctuaries spread throughout High Rock and Hammerfell.

Im pretty sure the DB is divided into Sanctuaries who are completely separate and dont interact unless absolutely necessary so "branch" didnt really seem appropriate

The entire Morrowind branch was made up of Daedra worshipers and was helped by fellow four corner worshipers.

I can find no evidence of that, all things point towards their worship of Sithis even in Morrowind, nothing is mentioned of Daedra however given that it is Morrowind i wouldnt be surprised if the Dunmer of the DB still held some praise for the Daedra but there is still no evidence of this

Bad wording on my part. Trying to describe the DB we see in Daggerfall the game.

Besides the fact that the Night Mother of the dark brotherhood in Morrowind is stationed in a Daedric shrine, right in front of a mehrune dagon shrine and the fact that all of the dark brotherhood operatives are in Daedric shrines except for the Mournhold chapter and a few single members and that we have clear evidence of the Dark Brotherhood associating themselves with Daedric worshipers?

We have plenty of evidence to support they did in fact worship Daedra in Morrowind because they are so closely tied to many of the cults which is shown in the Morag Tong questline.

Well my only explanation would be that Arena and Daggerfall have quite a bit of stuff that has been sort of "ignored" or even retconned in the modern games (religious buildings in Daggerfall had crosses on them for example) because i guess Bethesda didnt expect the series to be as big of a deal as it has become. But with the modern and more established lore there is never mention of Daggerfall being different than the other DB but then again any organization is bound to have some..."heretics" which is essentiallyw hat the DB is supposed to be for the Tong

Daedra worship in Morrowind is the norm and since Daedra (at least half of them) are put in a box with terms like evil we can assume that the DB used this to their advantage. SInce the Tong as the originals in MW we can safely say that the DB is very unlikely to be able to build sanctuaries in abundance due to the Tong's presence so using the already present Daedric Shrines is a smart move. This still doesnt mean they worshiped Daedra, i bet you would also say that since the Brotherhood has a lot of history with Mehrunes Razor that they worship Dagon? They value things that improve their ability to serve Sithis. I bet they view the Daedra as tools more than anything else especially since they see Sithis is an even more powerful force beyond Daedra and Aedra (which he is)