Un'ndexxor Xenocide

The Un'ndexxor Xenocide was a xenocidal Imperial campaign conducted by the Frost Lions Space Marine Chapter against the destructive xenos known as the Un'ndexxor.

Background
Just three years after the rein of Franz Makarov began, the Frost Lions were met with one of their greatest foes. A race of vile and destructive xenos was discovered by the Migrant Fleet lead by the Vasa Malum. Upon the moment of contact the xenos opened fire with their void weapons, the explosive power of which decimated the void shields of the Migrant Fleet. During the long campaign that followed, the Vasa Malum was boarded and assaulted. The Un'ndexxor xenos were unable to truly kill the Astartes whom defended their citadel vessel, but the disruption allowed the xenos to inflict a crippling blow with their weapons that led to the cascade failure of reactor defenses resulting in a reactor core breach.

The vessel was lost with nearly all hands as it made an emergency jump into the warp to avoid damaging the rest of the Migrant Fleet, with nearly all Astartes on board able to escape in time. Captain Oreas Julan, his loyal Honor Guard, and an unlucky few warriors who were more grievously injured than their brothers were the only Astartes lost in the battle. The Unrelenting Glacier was brought to lead the 3rd through 5th Companies in the total extermination of the Xenos species, Nidus Alvor ascending to the rank of Captain of the 3rd Company after Julan was lost. The conflict ended with the desolation of the Un'ndexxor homeworld and complete extinction of the species.