Omen 4: The Awakening
(1991)

Delia (Asia Vieira) is actually Damien Thorn's daughter. Baby Alexander (which is born 8 years after Delia) is Deliah's "twin" (!), which was implanted into Delia's mother Karen York (Faye Grant) after Dr. Hastings (Madison Mason) treated her when she fainted after the satanic dog killed the first nanny (Ann Hearn).

Delia's mother kills Dr. Hastings and the second nanny. She then sees Baby Alexander - he has "666" on his hand. She shoots herself.

(Yes, the end is that silly. Maybe that is the reason for the low vote result in the IMDB)

Thanks Tilman!

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Read more about this film at The Internet Movie Database.