Adeptus Custodes.

To reclaim humanitys lost colony worlds and protect them from alien threats, the Emperor knew he needed an army of super soldiers to do so. The Custodes was his first attempt. They far surpassed his expectations. To compare a Custodes to a Space Marine is like comparing a Space Marine to a regular human soldier. However there was a problem. The gene therapy to make a Custodes needed to start from the day of birth and took far too many resorces too make a massive army of them. So instead he used them as his bodyguards during the age of Strife.

They where the most loyal leagon to the Emperor, but they are also known for they're kindness to civilians.

After Malcador met with the Empress of Pleasure, the next she met was the Custodes. They attacked her until they realised the Emporer was apart of her. Half of them left to find a way to unfuse the Empress while the other half stayed to protect her until a unfusing methode could be found. However even though the Custodes do live far longer then normal humans, they're not imortal. The first generation of Custodes died before they could find a way to unfuse the Empress.

All generations after the first had no issues with the Empress, being just as loyal to her. In the main timeline, you'd never see a chaos Custodes because they're so loyal to the Emperor that they'd never fall to chaos. However in this timeline all Custodes are chaos Custodes since the Empress would want her personal leagon as strong as possible.

During the crusades and onwards the Empresses men would go to rich nobals under the Empier and ask to make they're new borns into Custodes. The parents may refuse but having your child be chosen to become a Custodes gives massive bragging rights against other nobal families. So refusal is rare.

The child is given special gene therapy and and training until they're 18th birthday. Then they are turned into futa's, given chaos powers and have sex with the Empress herself.

The Custodes get to have the largest harems of any leagons and are the most common to join the Empresses personal harem. Any offspring made by them are sent to be raised by the Custodes biologial familys. Some of them are even the Empresses offspring, giving the families even greater bragging rights.

Although it takes alot of resources and time to make a Custodes and even in the year 40k they're still the smallest human faction, their numbers have grown enough that it would be a waist to have them only guard the Empress herself. So now the main faction guards all of Terra and sub factions have broken off to do different jobs. With them around, the Empier is at it's strongest.