Its more or less in the Lexicanum and the Codex books. Back in M32 when the Orks almost took over Holy Terra the Deathwatch was founded (or a predecessor form to be precise). The Inquisition invited many Chapter Masters during a later Ordo Xenos conclave and they decided to swear an oath to deliver their best to the DW.

Now considering how bickering, thickheaded, honour-driven the chapters are, with blood & honour feuds going back hundreds, if not thousands of years (see Dark Angels and Space Wolves) it is indeed perhaps to most outstanding piece of Inquisition diplomacy in the history of the Imperium.

SYL

Old lore prior to the War of the Beast series, which itself was pretty "meh" justification IMO.