Propaganda

Extinction of the Species
It was generally accepted by the Vampire Elders (Viktor and Amelia), and all who heard it, that the death of Macrus Corvinus, the first Vampire would lead to an extinction of their species. Though this is inaccurate, it was not contested by the Elders out of fear. What if?

This also counted for William, though despite the war between the Vampires and the Lycans, William, who had been entombed alive, was not killed by the Elders out of fear of Marcus's reaction. Marcus had turned Amelia and Viktor in return for their military assistance in stopping William. Their agreement was not to kill William. Instead, against Marcus's wishes, they entombed him for what they hoped would be eternity. The Elder Viktor was concerned also that killing William would also result in the loss of all Lycans, his potential source of slaves.

The Uncontrollable Werewolves
It was spread from the Vampire elders that Werewolves, the forefathers of the Lycans, were uncontrollable, savage beasts. That they lost their minds to the creatures they had become. This is somewhat false, as proven by William after his release from imprisonment. He is able to coherently understand Marcus when spoken, and recognize the differences between friend and foe. He did, however have uncontrollable rage, and was unable to shape shift like modern Lycans, thusly trapped permanently in his Lycan form.

However, there is some credence to this idea, as quoted here:

"(To defeat) the very first werewolves. A dangerous and infectious breed. (...) But these weren't the Lycans we know, disgusting though your bretheren may be, they at least are evolved. No,these were raging monsters, never able to take human form again. It was only later generations that learned to channel their rage and William's appetite for destruction and rampage was insatiable. He had to be stopped. And so, once Viktor's army was turned. The legions of vampyres under his control tracked down and destroyed the animals then captured William and locked him away. Viktor's prisoner for all time."

The First Vampire
To the Corvinus Vampires, it was told that Viktor was the first Vampire. That the rest came later. This is not the case.

"Some history is based on truth, and others on deception. Viktor was not the first of our kind as you were led to believe, he was once human. The ruler of these lands. Marcus, he's the one. The source. The first true vampire.So the legend is true?Toward the end of his ruthless life, when the next breath meant more to Viktor than silver or gold, Marcus came with an offer, a reprieve from sickness and death, immortality. And in return, Viktor was to use his army turned immortal to help him."

The Death of Lucian
The Vampires believed that Lucian, the leader of the (Corvinae Strain) Lycan faction, was killed by Kraven.

This was actually a lie with several purposes. Together with a scrap of Lucian's skin as evidence (With William cell keys' likeness burned into it), Kraven was able to assume a trusted leadership position and a hero status among his kind, as a warrior and as a patriot. The other was cover up the existence of Lucian and his surviving pack, whom had in fact struck a deal with Kraven to overthrow the Vampire Elders.

It appears that Kraven was defeated in their battle, and offered life if he helped Lucian achieve his goals of overthrowing Viktor and Amelia.