The Argentar, also known as the Light of Power, is the physical manifestation of the River of Time combined with the Ocean of Oblivion. It usually is in the form of a silver colored fluid, but the power of the Four Pillars and the Keystone of Stephos combined can cause it transform into a more powerful form, entirely light.
Many of the Sollux were able to see into time, but Stephos mastered this art so well that the River of Time became one with him, even being one of the factors contributing to his immortality. When he sacrificed his immortality along with himself, some of the River of Time managed to seep through along with his light.
While the rest of the essence of the Heroes created the Ultras, the Argentar was lost to time until it was discovered by Ultraman King. Ultraman King granted it to a group of outcasts and weaklings, the remnants of the species that became Ultramen, granting them the power to gain forms suitable for surviving the dangers of intergalactic survival.
Argentar grants great power to any being that touches it, sometimes in such great amounts that the receiver dies from the sudden boost. Because it is the physical manifestation of Time, it has power over the bonds of time-space as well, but can only be used in that way by one who has the Keystone of Stephos as well as the four pillars. Due to its connection with the Ocean of Oblivion, it also has ending power, but that is kind of spoilery and will be revealed.....
It can revive as well, but this is considered dangerous due to the fact it can give too much power for a just-revived being.