The Ifukube Veil was a cloud of high-energy ionized hydrogen, located in orbit of the gas giant planet Veletus V, and held in place by a combination of the planet's magnetic field, and the gravity of its five moons. It was named for 22nd century human astronomer Kenji Ifukube.
The Ifukube Veil was created millennia prior to the 23rd century, following a Tholian sectarian war, when the life energy of billions of Tholians were released into the gravity well of Veletus V. Though this plasma coalesced into a single mass, the energy signatures of the individual Tholians were preserved within. The Tholians later abandoned the Veletus system, and attempted to eradicate all racial memories of this conflict.
In 2266, the Federation claimed the Veletus system, and began construction of a space station, designated Deep Space Station M-33, to study the Ifukube Veil. A number of satellites were positioned around the Veil as part of this study, as well as in an effort to tap the Veil's enormous energies.
Shortly afterwards, a number of Tholian separatists, calling themselves "the Children of the Lost Ones," attempted to merge with the Ifukube Veil in the last moments of their own lives. The Tholian Assembly claimed these pilgrims were in fact pirates, and reached an agreement with Commodore Julius Merrill to detain and return these "Outsiders". One of these detainees, Tholian Mage Naskeel, was able to communicate to Starfleet the true nature of the Ifukube Veil, and its importance to the Children of the Lost Ones. Negotiations were then initiated between Commodore Merrill and the Tholians to determine the future status of the Veil. (TOS short story: "Fracture")