Quote Originally Posted by Cheesegear View Post
Deathwatch aren't a single Chapter.

Rather, they're a special ops team of Space Marines from all Chapters who volunteer or get drafted into the military arm of the Ordo Xenos. Typically they get sent out into frontier space (the Halo Stars) along with Imperial Guard and Explorator Fleets to further the Imperium's reach. But they can be sent into warzones where a xenos threat is particularly strong.

As 'The Deathwatch' are made from different Marines from all sorts of different Chapters, each squad can learn from every member of it's team to learn different tactics to overcome different situations. Then, when their Vigil is done, a Marine will return to his Chapter with a whole bunch of new knowledge of how to fight aliens, and also have valuable contacts with other Veteran Marines from other Chapters.

Essentially, the Deathwatch are Grey Knights. Except the Deathwatch use Science and Skill instead of Psykery and cheating.

The only non-Veterans in a Deathwatch Vigil are Librarians. A veteran Librarian is far too valuable a resource for any Chapter to give up - even to the Deathwatch. So they typically send Librarians that aren't exactly the best in the Chapter. But, because of their inexperience, 'Deathwatch Training' is invaluable to a Librarian and if he survives his Vigil, he kind of comes out the other side being the ultimate badass.
I mean which of many chapters are playable in Death Watch, like Salamanders (though they may have pet peeve on other spacemarine gunning down imperial citizens) or Imperial Fists.