Alicia Dominica was once the leader of the Brides of the Emperor, the bodyguards of Goge Vandire. However, after she renounced her loyalty to him, she went on to reform her order into the Adeptus Sororitas.
Official Canon Edit
Not much is know of Dominica's service to Goge Vandire prior to Sebastian Thor's invasion of Terra. She was met by the Adeptus Custodes in secret during the siege of the Ecclesiarchal Palace and initially refused to renounce their oaths to Vandire. As such the Captain General bought her and five of her fellow sisters to the Golden Throne and the Emperor with the Imperial Palace.
It is unknown what exactly transpired in the throne room but upon returning to Vandire she immediately declared him a traitor to the Imperium, a heretic and declared his execution was long over due, although Vandire, who was completely out of his mind at the time, did not fully realize the gravity of the situation even as he was decapitated by Dominica.
Following this Dominica's role in the assassination of Goge Vandire allowed the Bride of the Emperor to be rolled into the reformed Ecclesiarch as the Adeptus Sororitas, essentially bypassing his own decree that the Ecclesiarchy would maintain no 'men under arms.' She would go onto lead the Sororitas for seven centuries, seeing it divide into two Convents, before dying during the Scourging of Frideswide's, where she allegedly suffered over a hundred Lasgun wounds before being finally shot through the heart. Prior to her death she was made a living saint, and after dying her famous speech to Vandire was etched onto her sarcophagus.
TTS Canon Edit
Kitten was the one who brought Alicia to meet the Emperor. As it turned out, he didn't need to do much in the way of convincing her, as the sight of the Emperor's decaying corpse made her sexually aroused (it's inferred that all Sisters, and members of the Ecclesiarchy for that matter, are sexually attracted to the Emperor, much to his dismay). Shortly after Kitten told her that Goge Vandire needed to be stopped, she simply agreed to it and beheaded him.