Titans.

Half way through the Age of Strife, their was a civil war on Mars. The Mechanicus had not yet made their rule of not making new technology. So to end the war, they created the first ever Titan. A massive robot with emence power. It singlehandedly won the war for the side that made it.

To piolet a Titan, you need a human with an exceptional level of willpower. These piolets are known as Princeps. Piloting a Titan takes a huge tax bothe metally and physically of the Princep. Not only that, piloting a Titan is very addictiving. So much so a Princep may keep pushing past their limits just too keep feeling the sensation. So they are aiden by Titan Crew's. These crew mates lesson the strain on the Princep and maintain the machinery.

When the Empress of Pleasure retook Mars, she was in awe of the first Titan and demanded the Mechanicus make more. Not only that, she would make all Titans Chaos Titans too increase their power and as too not waist all the resources needed to make a Titan. She also made it a rule that at least half of a Titan Crew must be of that Titans Princep prefered gender so they may have a harem as reward for their service.

Titans are not Dark age of Technology tech, so the Empier can make as many Titans as they want without the need of finding the sciatics. The only limit is the amount of resources the Imperium is willing to spend to make them.

The enemies of man tremble at the sight of them.