Shattered Imperium

Following the assationation of the last emperor the Third Imperium is shattered. Several claim the throne, the Imperium is falling apart, each faction claiming it's own small power base.

This setting covers the Official Traveller Universe from the Assasination of assassinated the Emperor Strephon, the Empress Iolanthe, the Grand Princess Ciencia Iphegenia, and the Aslan Yerlyaruiwo ambassador, by Dulinor, the Archduke of Ilelish, in 1116 through to the release of the AI Virus in 1130

From the introduction of the MegaTraveller Players' Manual:

"The time is the distant future. Humanity has made the leap to the stars; interstellar travel is not only possible, but common place.
The ruling interstellar power is the Imperium, a large, human dominated empire more than a thousand years old, encompassing 11,000 worlds in a region over 2,000 light-years across.
But while the Imperium’s thousand-year reign has created an illusion of strength and lasting power, continued expansion and internal pressure have combined to strain the Imperium’s ability to govern to the limit. The cohesive thread that has held the vast Imperium together has always been the person of the emperor, the one individual to whom all Imperial citizens ultimately owe their loyalty.
Now the emperor is dead at the hand of an assassin, leaving no immediate heirs and no clear successors. The Emperor’s nephew claims the throne; so does his cousin. Even his assassin, citing an obsolete and obscure precedent, claims the throne. No one knows whom to follow; someone is following each of the claimants. Several border regions have lost their faith in the Imperium’s ability to protect them and have declared their independence. A few neighboring interstellar powers, learning of the Imperium’s internal chaos, have begun to advance into unprotected frontier provinces.
The Imperium has been shattered ..."

Note: Much of the background material defined for the Shattered Imperium also applies to The Third Imperium setting. Much of the Imperium was not defined when MegaTraveller came out.