I have an idea for a wacky D&D 3.5 character. She is a human wizard, but, she is over 1500 years old. Is there any way that she can expand her own life span or make her eternally young? I was thinking that she somehow sucks the souls of people and uses the life force to be young forever. Is there a spell that does that or does anyone here have an even better idea?
I know of three ways of life extension.
The first is, when you get old, to have a druid kill and reincarnate you. This resets your age, but it also changes your race unpredictably, so it is not for the faint of heart.
The second is to become undead, most commonly a lich or vampire. This poses problems for player characters for multiple reasons; one of the problems is that you will get a high ECL cost.
The third is to get made into an "elan", which is a race that ages very slowly and never dies of old age. We're not told what the ritual is to become an elan, but it's available as a starting race, so you can just start as one.
I don't know of any way to increase your lifespan by consuming souls. Most games do not last for very many years of game time, so a DM might choose to house rule this in for you, figuring that it wouldn't affect anything about the story.
(I suppose you could narrate that your character has a ritual for becoming an elan, and that ritual requires consuming souls. So you'd get a druid to kill and reincarnate you, and then convert your new form into an elan? This would still require DM approval for you to have access to the ritual, but it doesn't seem implausible.)
On the other hand, it sounds like you're trying to create a deeply evil character. When someone tries to play an evil character, it creates hard questions for the group: do the other characters oppose evil? If so, why do they trust your character? Are their characters right to trust your character, or would your character kill theirs, given the chance? In short, why are your characters friends?
Often, when someone wants to play an evil character, they want to answer these questions as follows: "Well, the other characters have to be friends with my character, because we're playing D&D together. And, no, my character is totally evil and would kill their characters given a chance. I guess it's pretty dumb of their characters to be my character's friend, isn't it?"
Many DMs will feel that this is a mean thing to do to the rest of the group, and will simply forbid people from playing evil characters.