Posting the relevant page:
The daemon lord threw back its head in pain as a paroxysm of rage shook its body from the tip of its blackened horns to its splay-clawed feet. The dagger grew warm in Uriel’s hand, as though recognising a target for the lethal malice bound within its blade by unknown smiths of long ago. A shudder of unadulterated terror passed through the M’kar as it turned its gaze upon Uriel and saw the glinting dagger he carried. Its eyes widened in recognition.
“The shard of Erebus!” cried the daemon lord.
As much as Uriel wanted to strike back at the daemon lord for all the suffering and death it had caused, he knew that was not the role fate had assigned him.
He was the Sentinel of the Tower, not its Master.
Uriel hurled the dagger, hilt-first, towards Marneus Calgar.
The Chapter Master caught it deftly, the slender weapon absurdly small in his mighty gauntlets. But just as the Gauntlets of Ultramar were capable of great destruction, so too were they capable of feats of great dexterity. Held less than a metre from the daemon lord’s face, Lord Calgar lunged forward and plunged the ancient dagger into M’kar’s throat.
The effects were instantaneous and incandescent.
Blazing starfire spewed from the mortal wound dealt to the daemon lord, a flood of immaterial energy that raged in the air like a slick of polluted light. M’kar released its hold on Lord Calgar, who landed heavily on the ground before the dying daemon lord. Uriel ran to the wounded Chapter Master and, with the help of Pasanius and Learchus, dragged him away.