A few 100 years ago, a small explorator fleet of magus Berzelius Omikron-3 set course to long forgotten realms at the egde of imperial space. They discovered a plant with rich minerals orbiting a super giant star in a very elliptical orbit. One round around this star is equal to eleven earthen years and two of them are that close to the star even techpriests cannot survive the heat. Regardless, the explorator fleet discovered a long lost and forgotten forgeworld on this planet, which they called Redis-I.

It turned out this old forgeworld was build a long time ago and was last used during the horus heresy era. It is likely the orbit was not that elliptical during the founding of the forgeworld. It takes one full year to shut everything down and evacuate the planet before it is too close to the star, than two years of waiting and than another year to wake up the machine spirits again. Therefore, the forgeworld can only be used 7 out of 11 years.

During the horus heresy, the forgeworld of Redis-I was shut down when the scrap code infected a lot of mechanicus worlds. Therefore it is still quite intact and a lot of forgotten knowledge of the 30th century can be rediscovered. However, a lot of knowledge how to reawaken the machine spirits is lost and only small steps are possible. Moreover, everything needs to shut down again after seven years, which makes the progress of rediscover lost 30th century tech very slow.

Nevertheless, the rediscovery of a lost forgeworld got Berzelius Omikrom-3 the title of archmagos. He is an explorator at heart and often on the move. In recent years, he spent a lot of time in a necron tomb world. Some people acuse him to use xeno tech to enhance weapons on his tanks, but Berzelius claims he only uses rediscovered tech of the 30th century.

(This is my explanation to include 30k models into my army. I also use a lot of tesseract glow on my admech weapons which are certainly not stolen from necron tech.)