In the first film
His main goal throughout the series was to father the Antichrist, corrupt world governments, and bring about the end of humanity through the Apocalypse. The Devil only appears twice in the series, giving the form of a large black dog. In a spectacular scene, the Devil kills a priest who knows of Damien's unholy lineage by impaling said priest through the chest with a church spire when Satan locked the church doors during a spectacular thunderstorm.
The Devil also notably kills Jennings, a reporter, who vowed to kill Damien if Robert Thorn didn't comply. The Devil spectacularly killed Jennings by causing his truck to roll backwards down a slope in Israel, decapitating the man.
In the second film
The Devil is once again protecting his son and ensuring his success by killing all who know of his son's identity. The Devil kills a doctor who becomes suspicious upon realizing Damien's DNA is that of a jackal's, and he kills him by smashing his elevator down to the ground floor.
The Devil makes a prominent appearance in the form of a black raven in this film by attacking a journalist who was on the verge of making Damien's identity public; the Devil viciously pecks her eyes out, forcing her onto a road where she is struck dead by a truck.
In the third film
He is referenced many times by his son Damien in the third movie, and Damien points out the Devil in the depths of the river and says that he visited him first when he was six.
Damien's disciples empower Satan by committing another Massacre of the Innocents, recreating the Biblical events to ensure the next Christ will be killed too. Despite all the infanticide they commit strengthening Satan, they fail to find the real Christ child.
The Devil's minions are the Hellhounds, who attack the priests sent to kill Damien because they know of his unholy past. The Devil is felt rather than seen, manipulating situations and killing everybody who knows Damien's identity in gruesome supernatural ways.
Although not dead, the Devil is depowered and suffers a major defeat when Damien is killed.
The Devil is revealed to have conceived two children, one who is Damien, and the other a girl, to further his lineage.
The Devil is a cruel tyrant, furthering his selfish goals to conquer humankind in the most sadistic way possible, through mass murder and genocide. He likes psychologically tormenting his victims, upon making them feel secure until the final moment when there is no escape. He can control the weather, punch holes in reality, and displays formidable power. For such a major antagonist, he is not seen at all unless he takes on animal form, often that of a black hound or raven. He is completely ruthless and merciless, and inspires fear in his minions and enemies alike.