Page "The Seer (Charmed)" Paragraph 6

Unknown to Cole, the Seer engineered an attempt on Phoebe's life.

She tricked Kurzon, a longtime rival of the Source, into going after Phoebe with promises that he would be crowned as the next Source.

The Source was presumed dead at the time, and it was understood killing even one Charmed One meant automatic succession to the throne.

When Cole found out about this, he threatened to kill the Seer if she ever betrayed him again.

On the day before her wedding, she unearthed a Lazarus Demon and ordered him to attack the Charmed Ones as a part of the Source's plan to sabotage the ceremony.

She helps trick Phoebe into marrying the Source in a dark ceremony ( Phoebe and her sisters thinking it was a private ceremony ) and helped Cole slip Phoebe some chocolates enchanted with a tonic she brewed up.

Both of these actions ensured that Phoebe would conceive a child that would be the Source's demonic spawn and contain nothing of her or Cole.

Otherwise, the child would have been Cole and Phoebe's child and thus would have been born as a powerful good witch similar to his mother and aunts.