Fell Bats are not listed in the available units for the Lahmian army trial list (Warhammer: Vampire Counts, Page 79). Switching the rule set to Lahmian reflects that special alternate list, and so it doesn't have them. If you're just looking to make a regular VC list with a Lahmian vampire you should create a standard rule set army and select the Lahmian bloodline option on the individual vampires you take.
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