Quick answer: enter the Forest as Mage, find An Upsidedown Individual, take the blood-related route, and complete the resulting encounter.
Before the run
Use Mage and avoid locking an incompatible subclass before the event appears.
At the encounter
Follow the vampire route and finish the fight. Dialogue wording and secondary rewards may change during Alpha, so confirm the live prompt instead of relying on an old verbatim sequence.
After unlocking
Learn the subclass's self-risk and sustain loop in normal encounters before using it for an event or boss route.