A lot can be forgotten in 3000 years. Men and women who are powerful in their day fade into memory after 5, 10, 100 years. There's no shortage of examples of this throughout history.
Vast empires with cruel emperors sprawled across the Earth in the distant past, having left nothing but scattered ruins today. The memory of those who founded them at the price of tons of blood has dwindled so much that often even historians don't know their names.
1. Tiglath-Pileser I or Teglath-Phalasar I
One of the most terrible monarchs to ever rule was the Assyrian kin Tiglath-Pileser I. All memory of him has paled to the point where historians aren't even sure of his exact name.
In his time, that name inspired horror. When he ascended the throne he was the head of a small crumbling country but after 40 years of rule he left behind a tremendous empire, stretching from the Indian Ocean to the Black Sea. The first thing he implemented as king was the reinstating of human sacrifice.
During his reign, Assyria became known as the "Kingdom of Skulls" because every settlement he conquered he decorated with the thousands of heads of the dead, impaled on stakes. He also passed a special law - considered to be brutal and disdainful of women even at that time - that degraded women to the level of livestock. Tiglath-Pileser I died in 1076 B.C. and the entire ancient world breathed a sigh of relief.
2. Suppiluliuma I
He was the mightiest ruler of the Hittite Empire. Even though he expanded it from the Caspian Sea to the Balkan Peninsula and Palestine, history remembers him for his unsuccessful attempt to raise his son to the Egyptian throne. His heir was killed and the Hittite king began war with the Egyptians. During his campaign he found his own inglorious death.
History does not recall the name of the most renowned ruler of ancient Ethiopia. Only his initials - GDRT - are known from surviving Greek inscriptions. Scholars presume his name may have been Gadarat, or something similar. He reigned over the ancient kingdom of Aksum, which at the time encompassed the lands of today's Sudan and Arabia, besides Ethiopia. In pre-Islamic Arabic inscriptions found in modern day Yemen, Gadarat is mentioned as the king of kings, before whom everyone trembled.
4. Mahapadma I
The first empire of India was formed by Mahapadma. He was hated by every nobleman on the subcontinent due to the fact that he came from the lowest caste. His father was a simple barber. This didn't stop him and he united the greater part of India. Another reason why he was hated was for his extreme cruelty, as he systematically wiped out the higher Indian caste. For these reasons, the surviving noblemen did everything in their power to make sure he was forgotten after his death.
History remembers Attila and his Huns but has nearly forgotten about the Alchon Huns and their most infamous ruler Toramana, who wrought the same devastation to India and Central Asia as Attila did to Europe, while the country he founded continued to exist long after his death.