Wherefore Art Thou, Mercurio?
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After witnessing the beheading of your sire, and the outburst from Nines Rodriguez, Prince LaCroix will dictate that you go to Santa Monica and seek out an agent of his named Mercurio. You're provided with a haven above Trip's Pawnshop, so the first thing you'll want to do is explore it a bit before heading to Mercurio's. In fact, Mercurio has left you a note on your desk that tells you the password for your computer ("sunrise") and mentions that his address is in your email. It also gives you the freedom to snag the $100 left for you in the desk drawer.

Once you've read the email and found that Mercurio resides at 24 Main Street (apartment #4), grab the Normal Watch off the floor of the bathroom, the Pill Bottle from your bathroom cabinet, and the three Blood Packs from the refrigerator and head outside. Grab the paper on the floor just down from your apartment to find that a murder at the Santa Monica pier has made the front page, which will activate the Carnival of Death quest. Don't worry about searching the other apartments or your mailbox just yet - you'll be returning to them later.

Once outside, your first encounter will be a bum in the back alley. I recommend keeping your money to yourself and simply draining the bum of the vast majority of blood in his veins... but it's your call. Once you leave the alley, a short cutscene will follow that shows a bloody Mercurio entering his apartment building. Head down the street and into 24 Main.

A trail of blood leads you to apartment #4, where you'll find Mercurio lying broken and battered on his couch. Speak with him to learn that he was jumped by some thugs down by the beach while trying to purchase some Astrolite explosive. He'll then ask you to retrieve both the explosive and his money, which will trigger the Surf's Up quest. Further dialogue with Mercurio will make you realize that he's in terrible pain and that you should find him some pain killers, triggering the The Pain of Being Mercurio quest.