The Da'at Yichud is an ancient Jewish mystical secret society that has designed and created inventions centuries ahead of contemporary times. Their practices are based on pure reason and are described as a way of understanding God through knowledge and natural law rather than, according to Set Roth, "supernatural bupkis". Their creations never had a purpose beyond the act of creation and were nothing more than a method of communing with God. As such, they were never intended to be used. However, one of their vaults was discovered, their gatekeeper was killed and looted by Nazi forces, who reverse engineered them to make their super war machines that were perversions and mockeries of the religious society's work.
The Da'at Yichud had hundreds of safe-keeps containing their knowledge and inventions scattered around the world, which kept them from falling into the wrong hands and each vault has a gatekeeper. However, the Nazis led by Deathshead found one of their safe-keeps, killed the gatekeeper, stole their secrets, and used them to win World War II. Even after the war, Nazi research into Da'at Yichud technology continued in labs such as the London Nautica to create and/or reverse-engineered new weapons such as the LaserKraftWerk.
Set Roth is one of the known members of the Da'at Yichud who survived the Nazis' purge. He joined the Kreisau Circle against the Nazi regime, helping them by finding one of the Da'at Yichud safe-keeps in the Atlantic Ocean. The extent of the artifacts the Kreisau Circle is obtained from that vault is unknown, as there were not many to be seen in it.
During the events of The Old Blood, it is heavily implied that the Da'at Yichud had existed during the era of King Otto and the document he found to create The Monstrosity was based on an ancient Da'at Yichud genetic technology.
During the events of The New Colossus, an investigation and examination conducted by Nazis scientists on captured Da'at Yichud technology showed that these craftworks are so remarkable that it looks like magic at first glance, but is, in fact, 100% scientific upon close examination.
One extraordinary fact about the Da'at Yichud is the invention of sophisticated technologies such as anti-gravity engines, directed-energy weapons and mind control devices during preindustrial eras. A prime example is the vault in New Mexico which contains research on anti-gravity technology built before European colonization.
Communion with God is thought to take place through dreams, and enhanced by hallucinagenic drugs.
Despite a good amount of their technology falling into the hands of the Nazis, the Da'at Yichud's advanced technologies still can resist the Nazis. Many Nazi reports show that a lot of Nazi scientists, engineers and technicians perished from handling excavated Da'at Yichud artifacts improperly. One officer bled out from touching an artifact; a technician was disintegrated after dropping a Da'at Yichud artifact on the ground and the person stood next to him got a third degree burn. This made the Nazis conduct a manhunt on Set Roth, the only known living member of the Da'at Yichud and force him to work for them in an effort to prevent these incidents from happening again.
However, Set was broken out of Camp Belica by B.J. Blazkowicz and the Kreisau Circle and has formed a strategic alliance between what remains of Da'at Yichud and currently active resistance forces, acting as their main scientist, engineer and archeologist.
The Nazis began to suffer setbacks by 1960, as Set Roth gave the Kreisau Circle and the American Resistance access to untouched safe keeps such as the one beneath the Atlantic, leading to the destruction of Deathshead's Compound and the stalling of Nazi scientific advancements.
- Cairo (Helga made discoveries there, when she was first discovered by Deathshead)
- Istanbul (possibly, Helga discovered documents that lead to Deathshead's technology there)
- Atlantic Ocean Trench (where the upgrade for the Laserkraftwerk can be found)
- Roswell, New Mexico (Set Roth mentioned that there was a cache near the infamous location, which was given to the US Government before the Nazis won the war.)
- Da'at Yichud roughly translates to "knowledge of the unification" in Hebrew.
- Ironically, Jews are deemed by Third Reich as enemy of the state and are being exterminated, but the Nazis actually achieved world domination by using Jewish knowledge and technologies.
- This runs contrary to the historical Third Reich's tendencies to scoff upon "Jewish science", most notably the works of Albert Einstein. However, it's perfectly in line with the historical Third Reich's secret nuclear weapons program, which depended upon Albert Einstein's theories.
- The Da'at Yichud seem to pay very large attention to aesthetic detail, with most of their facilities and technology being seemingly gold-plated and overly decorated. The Nazis on the other side, are very practically orientated, and all their technology is clear from decoration. However, this 'aesthetic' may also have practical purposes as well, modular, improved shields, stealth or even stealth technology. Better at standing pressure of remote locations such as bottom of the sea, or deep under ground. The German knockoffs are good, but true Da'at Yichud technology tends to be much stronger.
- The safe-keep used by Deathshead was described by Set Roth as "small" and "tentative".
- Much of the Da'at Yichud's technology, including the contents of the Da'at Yichud Safe, strongly resembles that of the Dwemer (Dwarves) from Bethesda Softworks' franchise, The Elder Scrolls.
- Da'at Yichud as a story element appears to be a science-fictional re-imagining of the historic Judaism-derived transcendental tradition of Kabbalah. While the historical practices of Kabbalah seek an understanding of the divine and the infinite through study and esoteric disciplines, Set Roth describes Da'at Yichud as seeking communion with God through advanced scientific and technological study.