Deathwatch
The Deathwatch is the chamber militant of the Ordo Xenos and is composed of the most badass Space Marines from every chapter who are deployed according to their skills and which specific kind of xeno they are the most experienced at facing. If they have a devastator squad composed of ten heavy bolter equipped Space Marines who are sent to fight the Tyranids then you can bet that those heavy bolter equipped marines will be the best shots with a heavy bolter the inquisition could get their hands on and know everything there is to know about fighting the space bug lizards.
Their armor is painted black save for one pauldron which remains painted in the chapter colors to avoid pissing off the machine spirit, and another superfancy one which is silver. Surprisingly this paint job actually manages to look really badass even if it ends up being totally pointless with the Black Consuls and the Raven Guard. They get their shit done and get it done quickly and the only conceivable reason that they don't have their own codex is that they tend to be deployed in singular squads rather than fighting large scale battles. There are a few fan-made codexes for them as well as an RPG where they star as player characters though.
Just a single squad of 10 Deathwatch members will royally fuck up the shit of their target. A full deathwatch company's arrival typically spells the end of whatever unlucky xeno son of a bitch is on its receiving end. And if the shit has hit to the fan to the extent that one hundred deathwatch members have to deploy to one battle and organized into an actual company, that probably means that the Imperium is going to be sending a lot more than just the marines.
Deathwatch (RPG)
Fantasy Flight Games made an RPG about them, called, well, Deathwatch. Its supplement Rites of Battle has random chapter generation rules, which are used by fa/tg/uys to make a shitton of custom chapters.
What's this flub-duggery then?
The easiest way to get Deathwatch on the table without screwing around, unfortunately, really only applies to users of Codex: Space Marines. Sorry Space Wolves, Blood Angels (BA Sternguard or Vanguard'counts as' work just fine, add a Tycho 'counts as' for his rage against Orks and you've got a pimpin squad of bad-ass alien hunters.) and all the others, y'all are too awesome already.
Construct your Deathwatch Squad. When it comes time to use them in an army, count them as Sternguard or Vanguard vets. Voila. Deathwatch. No pissing around. If you still feel as if this isn't enough, increase their Ballistics skill, Weapons skill, Initiative and Leadership by 1, give them preferred enemy, increase their point cost somewhat and ask your opponent if you can use them.
Actually you can use Black Templar because the champion can take a vow that gives the entire army something similar to Preferred Enemy. Sure it won't be the same, but it's legal and you won't have to ask for permission. 6th Ed FAQ changed that vow, now it just gives Rage.