After Mercer Frey's betrayal of the Thieves' Guild is discovered by the Thieves' Guild members, speaking to Delvin Mallory yields the following conversation:
It's like the Dark Brotherhood all over again.
But from what I've seen, it seems that Delvin has a good relationship with the Dark Brotherhood, and a good relationship with their leader, Astrid, at that.
So what does he specifically mean by that line. Does he mean that at some point in Skyrim, someone in the Dark Brotherhood betrayed another in a serious way? Was it something that was done towards Delvin?
Best Answer
It's not entirely clear what he means by this. The Elder Scrolls Wiki article on Mallory has this to say:
That same article elaborates a bit on Mallory's relationship with the Brotherhood:
I did some additional research on the people and organizations involved, and if we just limit ourselves to Skyrim, then it's really not clear. It could be the theory from the wiki, it could be a specific betrayal we don't know about, or it could be a general reference to murder being more the Dark Brotherhood's domain.
Possible Oblivion Reference
However, as santyclause pointed out, a somewhat similar situation occurred in the Dark Brotherhood plotline in Oblivion. Since Mallory was a member of the Brotherhood, it's possible that he's aware of this history.
In Oblivion, Mathieu Bellamont infiltrates the Dark Brotherhood with the express purpose of destroying it as revenge for the murder of his mother. He not only lives among the "family", but becomes one of their brightest stars. He ascends to a position of leadership, as Frey did in the Thieves' Guild, and uses this position to cause the deaths of other members of the Brotherhood. This has parallels to Frey framing Karliah as a traitor.
These events precipitate the fall of the Dark Brotherhood, leading to the sorry state it is in at the start of Skyrim. Mallory may be worried that the same fate awaits the Thieves' Guild.