Even before Level 1, there's a lot of things that could possibly happen to a character that will have an impact on their adventuring career once they finally decide to head out into the bigger world. One is that they could come across various nefarious creatures and be turned, at least partially, into a bit of a nefarious creature themselves. How they balance these new powers with the person they once were can be an entire character arc. It's also what we get here in Gothic Lineages, a new Unearthed Arcana for Dungeons & Dragons.
From the post:
Transform into a creature of the night in today’s Unearthed Arcana! With the playtest options in the PDF below, your D&D character can become a dhampir, a hexblood, or a reborn.
We’ll release a playtest survey on this website soon. When we do, please let us know what you think of these options. Your feedback can help shape future D&D books!