Marian has it right. At the start of the quest, when you are at the portal with Serana she will explain that in order to be granted access through the portal you have to be undead i.e. a vampire. If you protest, the other option is that a portion of your soul will be trapped by Serana in a Soul Gem. This will reduce your overall stats. If you are a higher level player with a good amount of enchanted gear, the second option isn't that big of a deal. Once the quest is done, your soul is made whole again and your stats return to normal.
Also, as an FYI there is a cure for Vampirism that can be obtained (starts another quest) through Falion the wizard located in Morthal.
“A world is supported by four things: The learning of the wise, the justice of the great, the prayers of the righteous and the valor of the brave. But all of these things are as nothing, without a ruler who knows the art of ruling. Make that the science of your tradition!”
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Reverend Mother Gaius Helen Mohiam, from the novel Dune
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