"No one can stop a Shadow Game" explains a lot actually.
OOC//One of three things is going on:
1) 1,000 years is probably a mistranslation, although how one mistranslates 1,000 with 6 is beyond me.
2) We have yet again encountered another Takahashi-ism, where the author just can’t keep his story straight; or, he had other plans for Dark Malik, but just didn’t have time to expand on them.
3) Instead of being born from Malik’s anguish, the entity that alleges he originated during the initiation is actually a spirit that was born 1,000 years ago. This parasitic spirit has been passed down from Ishtar Heir to Ishtar Heir through the Rite of the Tomb Guardians, and even when his spirit has passed to a new host, his malevolence lingers (which is why Mr. Ishtar is probably as cray-cray as and delights in pain as much as his son).
This might explain why Dark Malik has knowledge that Malik doesn’t, such as the ancient language to awaken Ra. He may say he was born of Malik’s anger, hatred, and vengeance, and he does call Ishizu his sister once in a while. In reality, he’s like the alien from John Carpenter’s The Thing, able to assume characteristics, even thoughts, of his host, but in reality, he’s more ancient than anyone else realizes (or he remembers).
I’d always interpreted it as being a mistranslation of “millennia of hate” or something like that—basically saying hate that spans at least 1000 years.
And that Dark/Yami Marik/Malik, being the embodiment of the Ishtar’s hate for the abusive practices literally carved in their skin in the name of the pharaoh, is a symbol of their revenge. The hatred that he bears in the modern day has been built up over thousands of years’ worth of abuse, passed down the family line for the pharaoh’s sake.
Therefore, Yami Marik is setting himself up as a symbol of the Ishtar clan themselves… and he (along with regular-Marik himself) assumes that the whole clan has been doing this out of a twisted, enslaved form of duty rather than actual free will—since that’s the way his father raised him. I don’t think he’s literally claiming that he, Yami Marik, has been waiting a literal thousand years for revenge; he’s exaggerating in a poetic way. Well, as poetic as this maniac can get…