Removed from the Dragon Age entry, for being way too much Wall Of Text for a single example:

• To clarify: The Tevinter Imperium once ruled much of Thedas (the continent that the game takes place in) and was ruled by the Archmages, with the Archon as their leader. After their failed invasion of the Golden City in the center of the Fade, the Maker cursed the mages and turned them into darkspawn. So began the First Blight. The Blight lasted for hundreds of years, and the Imperium was gradually whittled down to a fraction of its former size and glory. The first Exalted March, led by Andraste, Daughter of the Maker, only served to weaken it further, despite winning by treachery within Andraste's own ranks (from her husband, no less). The Imperium now only has a few isolated cities left, and is slowly become overcrowded as refugees from the darkspawn and the qunari, the latter of whom are in constant conflict with the Imperium for control of the north. Pretty much the only time the player encounters them is in a quest in the Elven Alienage in Denerim, where you break up a slave trade. The Imperium's mages are more than likely all maleficars, and they worship dragons, which is the reason that the Maker first turned his back on his children. See if you can remember all that.
  • The above is the version of Tevinter's history as recounted by the Andrastian Chantry, whose biases might be showing. At least one codex entry reveals that current-day Tevinter also worships the Maker, but is in schism with other Andrastian nations because the Imperial Chantry has male priests and a male Divine, and they do not recognize the Divine of Orlais. This situation makes it pretty clear that ancient Tevinter is a loose analogue for the Roman Empire, with current-day Tevinter representing the Byzantine Empire, as a schism developed in Christianity between the Western Catholic Church and the Eastern Orthodox Church.

Anyone who knows the game want to condense it down to something salient?

I trimmed the real-life section down a bunch. Note that not all rump states, or indeed all ex-empires, fit the description — in fact, most don't (as they didn't usually rule a substantial fraction of the known world, nor do they necessarily have long periods of decline).

• The technical term is "rump state" and includes any state considered remnant of a once more extensive power. It may have since lost areas due to foreign invasion, secession or civil war. Russia is perhaps the greatest present example, even though just Russia is huge. Egypt and Italy are not considered rump states, but Turkey is.
• An incredibly little-known fact: In the fourteenth century, Lithuania was the largest country in Europe, with Belarus, Ukraine, and parts of Poland and Russia all being territories of the Grand Duchy of Lithuania. The Polish-Lithuanian Commonwealth created a new state which came into being in 1569, ruling over much of the same territory until it was systematically dismantled by Austria, Prussia, and Russia over the course of twenty-three years.
  • In some ways, the Polish-Lithuanian Commonwealth became this after its demise; it's political structure was one of the big influences on the U.S. Constitution, and thus later Constitutions with similar origins. As well, Polish national identity and pride in their free country persisted up to World War I, when they finally gained independence after being swallowed up by Russia, Prussia, and Austria.
• China has essentially been going through cycles of unification and disintegration for the past few millenia. While it has often been a single empire for centuries at a time, there have been countless periods when it has been reduced to a multitude of smaller states and warlordships all vying for power. Most recently, China was fast on the road to becoming one of these under the late Qing Dynasty and the early years of the republic, culminating in the Japanese invasion in the 1930s; fortunately for the Chinese (less so for Tibet and other rim states), they turned things around.
• The United Seven Netherlands - also known as the Dutch Republic - and its successors from about 1672 onwards. Between 1602/9 onward, the Dutch Republic had been one of the major powers in Europe, dominating trade (with a trade fleet larger than that of all of the other European countries combined), with the VOC (the Dutch East India Company) being the most powerful in the world. At its height of power, during the Golden Age, it included colonies and trade posts in Indonesia, Sri Lanka, India, Ivory Coast, North America, Brazil, South Africa, Japan, amongst others. After 1672 (called the Disaster Year, or Rampjaar in Dutch, where it was attacked by a coalition of France, England, Munster and Koln/Cologne) a steady decline started, where the Dutch trading position was taken over by the English/British in a series of four wars from 1652-1784, and wars with France and internal strife slowly turned it into a minor power. Eventually, the Republic was put out of its misery by the French in 1795. Its successor, the (United) Kingdom of the Netherlands managed to hold on to the Indonesian colonies and Suriname for about a hundred years more, until the mid 20th century.
  • The (United) Kingdom of the Netherlands is itself an example of this, after it lost half its core territory following the Belgian Revolution.
• For centuries, the Emperor of Japan was tremendously vestigial, with warlords like the Shogun holding the real power and sometimes not even bothering to help keep up the facade of the imperial court - one emperor waited twenty-one years for there to be enough funds for an enthronement ceremony, and some emperors supposedly were forced to sell calligraphy to make ends meet.
• The Republic of China once ruled all of China proper, and even had claims on Mongolia, but after the defeat in the Chinese Civil War, is now little more than a rump state that controls just the island of Taiwan and a few islands off the coast of the mainland.
• Similarly to Brunei, it is a little known fact that Oman actually defeated the Portuguese Empire and conquered itself a respectable if small maritime empire in Eastern Africa and the Indian Ocean. Then the British came in.
• The Achaemenid Empire (aka the Persian Empire, aka modern-day Iran), at it's greatest height under Darius the Great, encompassed very nearly the entirety of the middle east.◊ With approximately eight million kilometers of territory, it's said to be the largest empire of ancient times. Very few know this nowadays.
• The Soviet Union could be seen as an empire, in which case the countries it helped create that remain even though the USSR has ceased to exist (e.g. Cuba, North Korea, North Vietnam) could be seen as vestiges of that empire.
• Averted by the Ottoman Empire, which seemed to be going this way until, in the early 20th century, they underwent some major reforms and started rebuilding their strength, credibility, and prestige at a feverish pace. Just before the Ottomans found out that they were sitting on at least 20% of the oil in the world, they entered World War I and got beaten, stomped, and set on fire. All that was left was the rump state of Turkey, the rest of the Middle East being partitioned between Britain and France.
• The Holy Roman Empire was, for a time, a significant force, but it decayed from the outset. It was partitioned after Charlemagne's death; the remaining Holy Roman Empire was made of loosely affiliated Germanic and Slavic duchies. However, strong leaders like Otto the Great, Frederick Barbarossa and Frederick II kept the nation under a sort of unity. The overly prolonged death came when the Habsburg family came to power. Since the electoral system for the emperor consisted mostly of southern German, pro-Austria dukes and bribed Habsburg cronies, the Habsburgs never left the throne, alienating the North German nobility. Charles V's attempt to centralize the Empire in the 16th century, but the Protestant Reformation put an end to that, as the Protestant Northern Germans saw it as a rallying cause against the Catholic Habsburgs and Austria. Religious tensions culminated in the Thirty Years' War, which brought apocalyptic destruction to Germany, reducing its population by 15-30%. The war ended in stalemate and the empire now existed mostly in name only. (Voltaire observed that, by his time, the Holy Roman Empire was neither holy, nor Roman, nor an empire.) By the time Napoleon liquidated it it was a confederacy. The Habsburgs themselves continued to reign in their Austrian and Hungarian dominions until 1918. Yet by then, the Empire was already a relic of a relic.
• Alexander's Greek Empire of Asia. Not only it was one of the largest empires ever made but his conquests helped spreading the Greek civilization into the East. Archaeologists still find countless of greek monuments and artifacts from that era in all Asia. Not to mention the many cities took his name. Alexandria in Egypt, was proved to be the most prominent one. His Empire collapsed shortly after his death as his successors divided in new Hellenistic Kingdoms.

I added the Klingon Empire. Arguably, ever since they made peace with the Federation, they've been in decline. With everyone going for blood and honour, and over half of their fleet destroyed by the Dominion, they are more than likely in a downward spiral.