[The Sword 88]

The rules for taking allies are in the Codex: DaemonHunters or the Codex:WitchHunters

I am fairly certain non-codex chapters such as DarkAngels, Blood Angels, and Black Templar cannot take allies in their lists.

You can take a seperate detachment using its own FOC if your oppent also want to take a seperate detachment with its own FOC too. In that case DA can be allied with any army that does not have nonhumans in it. So no Orgyns or ratlings.

But you cannot just put an Assasin in your army anaymore. That was in the 2nd ed I think