There are actually two variations of Maid Issis and Butler Syncall as seen in the following picture from alla's:
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Unfortunately all of my screenshots are gone so I don't have one of regular Maid Issis but she would use one of the familiar models from MM. Maybe someone has a screenshot... her vampire model is:
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Basically the familiar version should spawn during the day, and the vampire version should spawn at night. The vampire version should act just like any other undead mob in the castle, and the familiar should act just like the rest of the dark elves. Both have the same loot table, but IIRC you have a better shot of getting a fang from the vampire versions.
I haven't made it high enough yet to get to Mistmoore, but if the only model spawning is the vampire model that is flagged as living, it would just need some tweaking to follow the spawn/behavior patterns above.