When we first meet Cassandra, we learn right away that she's a powerful witch who can do all sorts of crazy things, like read peoples' minds and call them to her using magical spells. Whoa. She has a great deal of power because of a stone that she wears around her neck called the phoenix-stone. But, everything comes with a price, and it turns out that the phoenix-stone is also slowly killing her:
"The other women who burned. The opal is known as the phoenix-stone because the fires recur. Almost everyone who possessed it died by fire. One woman was struck by lightning. Another perished in a house fire; that was said to be an accident. But there were other women who set themselves ablaze. Madwomen, suicides. One woman left a letter behind. She said that the women she saw in her looking glass had driven her insane." (19.37)
The phoenix-stone gives Cassandra all of her amazing powers, but it's also what will bring about her destruction. Well, unless she gets a kid to steal it from her. That's why she ends up calling the children to Strachan's Ghyll, so she can pass on the cursed stone to one of them.
Cassandra—or Madama, as she's called by her servants—has spent her lifetime accumulating plenty of wealth and power. She lives in a lavish home with beautiful gems, clothes, and other valuable possessions, but she's ultimately very unhappy (kind of like Clara is with her wealth). This is due to the fact that she has very little love in her life. She's only ever had one friend—whom she ended up betraying by stealing the phoenix-stone from her—and didn't grow up in a loving home as a child. And, as an adult, she was never able to get anyone to fall in love with her. She enchanted a whole bunch of men into "loving" her but knew that their feelings weren't true:
"I had admirers, of course; with the power of the fire opal, I could make men fawn on me. But their affection was neither lasting nor true, and I quickly tired of them. It's queer, isn't it? When I look back over my long life, there were only two people who mattered. One was Gaspare and the other was Marguerite. He betrayed me and I betrayed her." (49.46)
In the end, Cassandra realizes that everyone can see through her misery, and her wealth and power haven't brought her any happiness at all. Just before she dies, she decides that she will do the right thing for once in her life, and she leaves her fortune to Lizzie Rose and Parsefall. She wants them to have a better, happier life than she did.