"The surface was solid but suspect, not unlike ice near the end of winter, with the familiar crunch and crackle of the pieces jutting out of the ground. Odd reflective pools of muddy liquid dotted the area, pools which moved and writhed and sloshed unnaturally in the damp cave. Peering into the puddles for too long might give the vague suggestion of an almost human form slurping and tossing inside the sludge. Soon after one might see the bright pinpoints denoting eyes, and a face wrought in agony."
The Baptized are a humanoid, swarming enemy and like certain forms of the Stillborn they cannot be permanently defeated, merely knocked back down into the muck from whence they came. Once knocked back they will re-spawn elsewhere within their pool and re-emerge once more if approached.
They only appear inside large pools of dried Marrow. It is solid enough for Dan to walk across, and distinct enough to be recognised as a place that will harbour these goopy, undead beings. They have no real nuance to their behaviour; once they have line of sight to Dan they will come after him until beaten back or until they hit something.
The inhabitants of the dried Marrow come in two forms, the larger of the two is no faster than its smaller counterparts but is more deadly and has a much bigger pool of health.
Fire is effective but watch your clarity.
With the right weapon the Baptized drop in one hit, however having a weak weapon could mean dealing with a group of surrounding Baptized which may take three or even four shots to put down, a potentially deadly mix.
The Marrow-men of the deep regions of the Mountain represent some of the gruesome and twisted ways in which Marrow can effect an organism (living or dead).
The Baptized were likely the victims of internal Marrow poisoning, either during the course of an experiment or in a misguided step along some blasphemous ritual, these people ingested Marrow and were eaten and transmogrified by the substance, changing them into an almost completely liquid state.
Their new forms were not fatal at first. These poor souls likely lived a long, natural life as a pool of muck, contemplating their own choices and existence as the horrors they had become, before succumbing to death. Now even some of those still wander, undead, inside the halls of the Mountain.
Morrow OST - Pathways Youtu.be