the book you want is #20, your memory has mixed the 4 stories, but the angel is mentioned multiple times across at least 2 of the stories.
20. The Primarchs
The Primarchs is a compilation of four novellas by different authors, each story starring one of the "sons" of the Emperor.[78] The novellas further develop these characters, who make multiple appearances in the Series: following the Dropsite Massacre (Book 5), Fulgrim, perversely empowered by his own corruption, reveals his true nature and future plans to top officers of his compromised Legion; during a Great Crusade campaign against the enigmatic Eldar, Ferrus Manus, already beset by unsettling dreams, is subjected by alien sorcerers to stark, portendous visions and warnings about his future and role in the soon to be revealed Heresy; with Horus' rebellion in full swing after the events of Isstvan V, a suspicious and isolated Lion El'Jonson accepts the newly revealed realities of the Warp and decides on an independent course of action for the Dark Angels in the unfolding conflict; around the same time, and plotting a typically indecipherable course in the expanding war, the twinned Primarch of the Alpha Legion is involved in a unique counterintelligence operation that extends the Legion's customary deceptions inwards.[79]