Morvenn Vahl is a living example of both a great warrior of the Emprah as well as someone who gives no shits about political maneuvering. As the Abbess Sanctorum of the Sisters of Battle and a member of the Order of the Argent Shroud, she was chosen for the position as the High Lords of Terra, especially the Ecclesiarch (currently Eos Ritira, a woman of a reformist bend), wanted to manipulate someone young politically
and probably also in let's just say, other ways- *BLAM* Heresy! Given what her order is known for, it shouldn't have been a surprise what she would have done after accepting. She's known for leading from the front, preferring to have her second-in-commands handle her duties on the Senatorum Imperialis, and managing to make the derptastic Paragon Warsuit look at least somewhat decent. She also has the Lance of Illumination, a Power Spear gifted by the motherfucking Custodes. If that doesn't make you believe she's doing the Big E's work, please go see your local Commissar for correction.
Not much is known about Morvenn's pre-Abbess life (at least, at the moment - we can expect Black Library to flesh out the details), other than the fact she's a member of the Argent Shroud and was a Celestian. The latter part certainly speaks for itself in what she's achieved, but arguably not High Lord material - especially with her Order's whole "actions speak louder than words" shtick.
But, come the death of the previous Abbess, Sabrina of the Ermine Mantle, the Ecclesiarchy decided to take advantage of the situation and decided to pick someone young that they could easily make a political pawn. Yknow, typical corrupt shenanigans. But for some Emperor-forsaken reason, they had the "bright idea" of seeking that pawn in the Order of the Argent Shroud, an order infamous among imperials for being both deathly quiet and ignoring orders. Whatever the case, they decided to select Morvenn Vahl for the position.
So it was that Vahl was rudely interrupted during the Reskgard Purgation, Ecclesiarchal delegates telling her of her new divine purpose in the name of the Emperor. Speaking up for one of the few times in her life (being of the Argent Shroud), she subverted expectations by saying, to effect, "I'll think about it".
As one in the Sisterhood does, she would enter her Mission's shrine to Saint Silvana to pray and meditate on the matter for 3 days. After this period, Vahl decided to accept the promotion but denied performing proper ceremonies and rituals for her inauguration in favor of someone immediately filling the vacancy left by her and heading for Holy Terra. This was the second and by now very obvious red flag that the Ecclesiarchy chose their new pawn poorly - whatever the reason, be it Big E himself ordaining her to or something else, she would become the only known Abbess in recent memory to be a military leader.
Refusing to remove her armor, she was a highly intimidating presence for Cardinals and bureaucrats alike, further disrupting the Ecclesiarchy's plans to have her be a pawn for them. She would also explicitly focus on military matters, such as reestablishing contact with other Orders in the wake of the Great Rift. Roboute Guilliman would also make her position a High Lord as part of his reforms, to which she just had high-ranking Order Dialogus sisters take the duties of. This proved even more devastating to the Ecclesiarch and all those who sought to use Vahl as a puppet, as not only was the position of Abbess Sanctorum no longer solely subservient to the Ecclesiarch's authority, but Vahl didn't even give them the dignity of her presence in the Senatorum.
Shortly after being made a High Lord of Terra, Morvenn Vahl decided that things were getting too boring and thus, as one does, launched a full fucking Sisterhood crusade with herself at the helm. Gathering forces from every major Ordo to strike at the traitorous worlds of the Philosopher's Belt, she led by example and fought alongside her fellow nuns with guns against the heretics. Oh, and she ignored any governors, generals, and admirals that contacted her, having her Order Dialogus and Order Famulous people handle them instead. Truly, she gives no shits about politics, only fighting for the Emprah.