The reason for this is that the temptation of abyssal power was too much for many of them to resist; despite the admonishments and threats of the Solars, many of the original uranians gave in to the temptation of Lilith. They are more fearsome than most Others and highly valued in the army of the Abyssal Queen due to their entropic ability, formed from a combination of Dark and Earth.
Similar to the magic of decay, Entropy takes this process one step further; the very presence of a Uranian is enough to degrade weak systems rapidly, making them age to death or destruction within a series of moments. Although sorcerers have some natural resistance to this due to their wielding of Magic, the targeted power of a uranian is often enough to overcome this, ageing many of those who dare oppose them to dust within minutes. This has lead to the nickname of the uranians as the Timekeepers, although the Chronals have nothing to do with them; instead, their ability allows them to destroy beings of presumed immortality.