They never knew.
To be fair, I never told them, because no one ever asked. Dumbledore knew.
He was a legilimens. He saw the visions I had whenever he looked into my eyes. That's why he exempted me from most meetings. He didn't have any real control over his legilimency, ever since his duel with Grindelwald. Grindelwald attacked him with a curse that damaged his control; that's why he wore specs most of the time. They blocked his legilimency. His occulemency barriers were destroyed in that duel by the same curse. Most people don't know but in order to control one's legilimency, one must also be an occlumens but the converse, to be an occlumens one must be a legilimens, is not true. Anyone can learn occlumency, except seers.
And I'm a true seer.
Thankfully, my daughter is not.
You see, back in the time of the Founders, Salazar Slytherin was an alchemy potions master. He wanted to create a mirror that would allow one to look into other worlds. He accidentally created a potion that gave a person true seer ability, permanently. Herpo the Foul thought this was a poison and forced Salazar's daughter to drink it. She became a seer, and so did her unborn daughter. After that, females in the Slytherin line were always seers, ranging from powerful seers to mild. My power is unstable; it seems after so many generations, the effects of the potion on our blood is wearing thin. My daughter has almost no gift at all, thankfully. The "gift" which is more of a curse, drove my mother insane.
I have the inner eye which through the use of crystal balls, tea leaves, and the like, enables me to have visions, glimpses, vague glimpses of the future of my students.
I saw it.
The rise, fall and rise again of Voldemort. I saw that Umbridge woman and her actions against the students. I saw parents of students die.
I saw students die.
That's the real reason I drink, to force the images of the students lying dead out of my mind. To dull the effects of the inner eye.
That's also why I predict someone's death each year. I've seen them die. I saw Harry Potter die but I couldn't very well tell him that he will come back and Voldemort will die once and for all. The wizard world already thinks of me as a fake; I cannot have them think of me as insane too. I saw Dumbledore die. I've seen the future, the death of Snape, Fred Weasley, Colin Creevey, Lavender, that Crabbe boy, that Metamorph girl and Cedric Diggory. I see them all dead.
So, I drink, to wipe those images from my mind, to forget the pain I feel when I see them dead, to forget the images I see of the future, a future where dark lords continue to rise and fall in a never-ending cycle. To forget the death of those cycles.
Thank God, my daughter does not have the gift; she'll never have to bear this burden.
Sybill Trelawny – Last Journal Entry – January 1, 1998