Ironguts. They're cheap, and not having to drink antidote in battle saves time, focus, and energy.
Clarity: Vampire casters of all kinds. be prepared for mental domination attempts
Freedom of movement/remove paralysis: Paralysis by vampires observed.
Ghouls with poison breath: Infect you initially, when the poison sets you die. Priority one healing in combat. Focus these spitting ghouls down with all your might as soon as possible. Antidote or competent healer's medicine most likely counters once poisoned.
Darkness if frequent by these undead forces - know your way out in the need for an egress. Be prepared to be blinded. Be prepared for bandits turned cursed spawn of vampires to be attacking your vitals sneakily from this darkness.
Logistics:The Shadowfell is not an easy place to get to or get back from. This will need to be researched - if someone with an arcane disposition has an interest and ability to determine how one might get there and back again. Suggested review of the library for research. Sorcerous Mummy might have indicated that the spindle, while not in the Haunted Halls, would be elsewhere - but I suspect the door or portal to the Shadowfell might be located within the Haunted Halls and just be difficult to find / access. Scouting and investigating this at the moment is, naturally, quite dangerous and caution and extreme preparation are suggested.
Possible connection to realm experienced by some guild members prior: Boatman and his coin. Additional information via this report
(// can read but do not comment, 2nd posting by me is also fine to read): https://forums.cityofarabel.com/topic/67757/letter-selwyn-seal-signifying-kalen-smith-sender
Possible dangerous individual that fits the bill for those wanting to obtain undeath: The Bowyer - seeking the spectral coins.
-Kalen.