-( Story related, cannot be missed )-
After tending to Aloysius, you'll hear that Priwen is planning to take siege of Pembroke under the misdirected guise that you and Swansea are behind the epidemic. Rushing to Edgar's aid, you find that he has been abducted with a blood trail leading upwards into the third floor (for non-American standards, the second floor, as they count the ground floor as that and the floor above as the first). Taking the elevator up into what appears to be an attic, you will be confronted by McCullum and treated to this challenging fight.
Like Mary, McCullum employs the battlefield tactically to his advantage in the form of the ultraviolet rays that will flicker on and off. He has two ranged attacks, where he will shoot one stake directly at you, or several stakes at where you have been or will be. Strafe/dodge to avoid these as best as you can while trying to get up close to him. The main challenge from this fight comes in the UV lights, which will often fall upon him making it impossible to attack him while they're on. As the fight progresses, they will spawn in a 3x3 grid akin to Mary's blisters, or even shine down upon him in a larger radius. Beyond that, he has a one-two-three combo and a running slice that are simple to avoid. The only threat that comes from this fight, as I can't stress hard enough, is the UV light, as it will drain your health AND your stamina by maintaining a position under it. The frames of invulnerability that comes from biting him will prevent any major damage, so there is that.
Defeating him, you will be given a choice to spare or embrace him. Clearly, embracing him is a bad thing this far into the game, as it will void the following:
The Hidden Trophy trophy in Vampyr worth 240 pointsContinue playing to unlock this hidden trophy