When you are ready and able to acquire a new familiar, you may choose to gain a stitched flesh familiar.
Prerequisites: Ability to acquire a new familiar, ability to cast three or more necromancy spells.
Benefit: When choosing a familiar, you may choose a stitched flesh familiar. A stitched flesh familiar appears similar to any of the standard familiars available, except that the stitched flesh familiar is obviously sewn together from many different creatures of that kind and, to a practiced eye, is clearly an undead creature.
A stitched flesh familiar is magically linked to its master in the same way as a normal familiar. A stitched flesh familiar uses the basic statistics for a creature of its kind, as given in the Monster Manual, except as noted below.
Hit Dice: A stitched flesh familiar has a d12 Hit Die and gains no bonus hit points from Constitution (since it is an undead creature). For effects that depend upon Hit Dice, use the master’s character level or the familiar’s normal Hit Dice total, whichever is higher.
Hit Points: Use half the master’s total or the familiar’s normal total, whichever is higher.
Attacks: Use the master’s base attack bonus or the familiar’s, whichever is higher.
Saving Throws: For each saving throw, use either the familiar’s base save bonus or the master’s (as calculated from his character level), whichever is higher.
Familiar Special Abilities: Use the Familiar Special Abilities table to determine additional abilities, just as you would for a normal familiar. Stitched flesh familiars do not grant their masters any of the benefits that appear on the Familiar Master’s Bonuses table. Instead of the noted special ability, a stitched flesh familiar grants its master the ability to control 4 more Hit Dice of undead than he is normally capable of controlling (both through the rebuke undead ability and through spells such as animate dead).
Source: Libris Mortis, page 30.