Compassion is reserved for servants of the Emperor: aliens deserve only our scorn.
-Inquisitor Czevak, Teachings on the Unholy
The Ordo Xenos is one of the three major Ordos of the Inquisition, dedicated to counteracting the threat of alien species. Their military arm is the Deathwatch.
The Ordo Xenos was created to investigate and catalog xenos species, and destroy any classified as a threat to the Imperium. This jurisdiction has been expanded to any members of the Imperium that deal with xenos, trade (or deal) in their technology, or seek to hide or protect hostile alien species from Imperial forces. Inquisitors of the Ordo Xenos are devoted to their cause (fanatically so, in many cases), and are among the greatest force that can be wielded against any given xenos species.
Armed with the best human and xenos technology known, extremely knowledgeable about their foe, unflinching in their mission, the Ordo Xenos can respond to any possible xenos threat. Their tactics vary depending heavily on the situation, species of xenos involved, and level of xenos influence on the local population. Where the threat is subtle they will use guile and stealth, wielding their power as a scalpel is used to cut out a cancerous growth. When the alien menace is great, the Inquisitor can enlist the aid of entire regiments of Imperial Guard and Space Marines. When the threat is too great for even these to handle, the Inquisitor will call on the elite battle-brothers of the Deathwatch. In extreme cases, entire planets are known to be destroyed through Exterminatus.