What they were called in ancient times is unknown.
What is a remnant?
Remnants are pure Mana Energy. They are usually man made through a process of condensing enormous amounts of Mana Energy into a single point. Once enough energy is condensed, it stays in a spherical shape. Once they collapse into spheres they start absorbing Mana energy into themselves. The process is exponential, as the energy level grows, the rate at which it draws in energy increases.
They are solid enough to be picked up with the proper gloves, but they aren’t truly solid. What would happen if you picked a remnant up with bare hands would depend on the element of the remnant. Fire for example would burn you.
They are more like the sun. The only thing keeping them from flying apart is their energy density and the fact that Mana energy attracts itself.
Remnants could theoretically be used as an energy source themselves, but once they get low on energy, the rest of the energy disperses into the environment.
What are remnants used for?
Scientists thousands of years ago discovered that massive amounts of Mana Energy can be condensed into a single point, which once enough energy is accumulated will remain in a sphere without a containment device. A bit like how a star stays a sphere once there is enough gravity pulling inwards.
While a Remnant composed of every element is theoretically possible, scientists generally created Remnants with a single Element because it was more cost efficient and more stable. It is rumored that a “Master Remnant” with every element at once can be created, but it may just be a legend.
Fire, Water, Wind, Earth, Nature, Metal, Dark, and Light Remnants were created, with an extreme cost.
Remnants are the driving force behind the Mana Assimilator. When all 8 Remnants are harnessed inside the Assimilator, energy of every element is drawn into the device, where it is swept up by the Assimilator before being absorbed into the Remnant.