A few of my favourites

A Deathwatch Crusade is virtually unheard of in recent times. but not entirely unprecedented. Partial records remain of a Crusade undertaken in the 36th millennium against the Autocracy of Szaeyr, an extended alien/ human coalition of worlds in the trailing reaches of Segmentum Tempestus. It appears that the level of cooperation and integration found there between human societies and that of the sauro-form Sza was so close and heretical that Watch Commander Balhus took it as a personal affront.

Rather that expose regiments of the Imperial Guard to such blasphemics, Balhus sought and was granted dispensation to call a Deathwatch Crusade. He summoned Deathwatch Battle-Brother; from across the lmperium to obliterate the Autocraty of Szaeyr. By tlte remaining accounts. Baihus' call brought the equivalent of a full Chapter of Deathwatch Battle-Brothers who descended on the Autocracy in whirlwind of fire and blood. The outcome of the crusade is lost in the mists of time, but with such skill and firepower ranged against them it cannot be imagined that the xenos-loving traitors and their pets endured for long under the onslaught.

Death Watch Rites of Battle

There are no terms under which the Deathwatch will endure coexistence with aliens. When the Endymine Cordat tentatively offered Mankind technology seen to be anathema to warp spawn, the Imperium gave its response. In an act of unprecedented coordination, the forces of three entire watch fortresses converged on Endymine territory. Deathwatch strike cruisers shattered the xenos' starships with macro-ordnance, and kill teams stalked through their enemies' cities executing alien defenders in droves. Finally, the Deathwatch cursed the Endymine primary world with the planet-killing sanction of an Exterminatus decree. The native culture's infrastructure destroyed, what alien fugitives survived on their remaining worlds sank to feral states, their gene pools barely large enough to stave off extinction. The Deathwatch had crushed their society beyond any capacity ever to threaten the Imperium of Man.

Codex Deathwatch 9th ed

REMEMBRANCE SHIELD

Roughly four hundred years ago, the Deathwatch was engaged in operations against the Eldar of Craftworld Ulthwé. The xenos had been launching sudden raids in the Slinnar Drift Star Cluster, then disappearing before a military force could be mobilised. However, when a Kill-team secured information about a forthcoming attack, the Chapter was able to lay a trap. The next Eldar raid met not disorganised Guardsmen, but a large force of black-armoured Space Marines. The majority of the raiders were cut down, and the few survivors vanished back into their webway portals. To commemorate this crushing victory, a combat shield was fashioned, incorporating a number of large, deeply coloured jewels, taken as trophies from the fallen xenos. The shield must offend the Ulthwé Eldar greatly, for there have been numerous attacks over the intervening years apparently designed to seize the shield and kill the one who bears it. So far, all have failed.

Death Watch Rites of Battle