Major spoilers for the three endings as well as the depicted NPC’s (Eileen the Crow) questline, so read at your own discretion (and maybe don’t if you’re planning on playing yourself). This’ll be a bit off the top of my head and rather lengthy, so if you’re not that much into the lore, the TLDR probably suffices too (also spoiler).

TLDR: It’s a reference to one of the three endings of BB where you become an infant Great One. The scene depicts the end of Eileen the Crow’s questline.

Bloodborne has three different endings depending on what you choose in the final interaction with Gherman and if you have consumed at least three Umbilical Cords: he can either execute you, thus severing your connection to the dream, making you “mortal” again and waking you up from the dream/nightmare, so you end up not remembering any of the happenings of the game and wake up at the beginning of the hunt; you can choose to fight him instead and depending on if you have consumed the three Umbilical Cords prior to the fight, you either kill Gehrman and take up his job when the Moon Presence (ultimately responsible for most of the major plot points of the game) comes down and anoints you this position; or you become an infant Great One yourself (cosmic, eldritch beings comparable to something like Cthulhu whose power many scholars try to achieve for their own purposes), thus surpassing the Moon Presence itself and achieving insurmountable power.

The comic is a reference to the last of the three endings where you become a Great One.

The Healing Church, an institution in Bloodborne comparable to a cult whose members “invented” the use of blood transfusions from Great Ones’ blood to heal any ailment, kept prescribing blood transfusions to the populus even though they knew that receiving too many treatments would end up turning you into a blood-crazy beast; this ultimately led to the events of BB by turning most of the population of Yharnam into beasts.

These beasts are hunted by hunters who were part of Gherman’s crew and later on Ludwig’s as part of the Healing Church hunters. Hunters would end up treating themselves with blood transfusions, though, and would also turn into beasts in time.

This is where Eileen plays a big role: she is a hunter of hunters, of whom there is only ever one at a time, who go out into the world and kill hunters that have succumbed to the beast frenzy caused by consuming too much blood.

The comic shows one of two different ways Eileen’s questline can end: if you helped her throughout her journey, she lays in front of the Grand Cathedral, bleeding out, telling you about an insurmountable foe that she was not capable of killing. That’s where you’d normally meet her, go into the Cathedral and kill the guy that almost kills her, and end her questline, shown in the comic. :)

Sorry for any typos or unclear information - didn’t have a bunch of time to write this down. :D Nevertheless, I hope that gave you some context for the comic.