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 were discovered 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. However, the Nazis led by Deathshead found one of their safe-keeps, stole their secrets, and used them to win World War II. Even after the war, Nazi research into Da'at Yichud technology are 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 and joined the Kreisau Circle against the Nazi regime, helping them by finding one of the Da'at Yichud safe-keeps in the Atlantic Ocean. It is not known the extent of the artifacts the Kreisau Circle obtained from that vault, as there are not many to be seen in it.
During the event of The Old Blood, it is heavily implied that Da'at Yichud has existed during the era of King Otto and the document he found to create The Monstrosity was based on ancient Da'at Yichud genetic technology.
During the event of the New Colossus, an investigation and examination conducted by Nazis scientists on captured Da'at Yichud technologies showing that these technologies are so remarkable in which it looks like magic at first glance, but it is 100% scientific in close examination. The most extraordinary fact about Da'at Yichud is that they create sophisticated technologies like anti-gravity engines, directed-energy weapon or mind control device during the eras, which concept of science still not in the mainstream norm like medieval and renaissance era. The prime example is the vault in New Mexico containing the anti-gravity technology is built before the Europeans arrived and subjugated Native Americans.
- Cairo (Helga made discoveries there, when she was first discovered by Deathshead)
- Istanbul (possibly, Helga discovered documents that lead to Deathshead's technology there)
- Deep Sea Trench
- 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 Nazi 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 aestethic 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.
- 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.