Familiars (CBN)

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A familiar is any creature, magical or otherwise, that augments the personal powers of a wizard. The familiar is more than a minor servant or hireling: it can serve as a pet and a confidant for a lonely necromancer, as well. At low levels, the find familiar spell can be employed to obtain a small animal that will transmit sensory information back to its owner. Animals such as the crow, raven, serpent, hyena, vulture, and weasel are most appropriate to the necromancer, who can presumably summon more powerful familiars than the average wizard by right of his or her secret knowledge.
Ordinarily, familiars do have a major drawback: because of the spiritual (necromantic) link with its master, when the familiar dies, the wizard must make a system shock roll to survive the ordeal while permanently losing a point of Constitution. Necromancers with familiars will take elaborate precautions to keep their familiars safe from harm.
Although these are the most common types of familiars available, it is also possible to attract more powerful, extraplanar creatures into a necromancer's service. With the proper sacrifice an evil wizard can summon a creature from the Lower Planes to serve as a familiar. Most often, the fiendish familiar will either be an imp or a quasit (as detailed in the Monstrous Manual). A necromancer (or an evil priest with access to the sphere of Summoning) may call an imp or quasit to him or her, with a 10% chance of success per level of experience.
Although the fiend often appears like a tiny, wicked creature, it could also take on a more dangerous or sinister appearance. For instance, instead of looking like an annoying 2' tall fiend (something akin to Jabba the Hut's maniacally cackling pet in the film The Empire Strikes Back), an imp could appear as a shadowy, winged creature with glowing red eyes (as illustrated in Captain Omar's introductory tale). The DM should feel free to embellish and tailor both the appearance and powers of the classic imp to suit the personality of its master and the mood of the campaign. Pizentios's “shadow imp," for instance, may have the ability to hide in natural shadows and darkness (gaining the ability to serve as an excellent spy), but it also has all the weaknesses of a normal undead shadow, such as aversion to bright light (see Pizentios's character description in Chapter Nine for more details). These essentially minor changes in appearance and abilities can dramatically change the way a party of adventurers regards the familiar (and its master as well), especially among experienced players who have perused the MM.
Depending upon the power of a necromancer, the “common" imp may not suit either his or her taste or image. Other fiends may serve as more interesting or appropriate pets. On the other hand, a more practical Archetype may prefer to have a nightmare as his trusted, magical steed and familiar. A few fiends (imps and quasits included) will willingly serve a mortal master, on the hopes that they will be able to collect the wizard's soul when he or she dies and present it to their overlords in the Outer Planes, gaining greater prestige (or in the case of an imp or quasit, a promotion in the infernal hierarchy).
In general, a necromancer may not have an extraplanar creature as a familiar with more than a third as many hit dice as the wizard's levels of experience. Thus, it would take at least an 18th-level necromancer to attract a 6 HD succubus or nightmare familiar. In addition to the usual telepathic rapport, the fiend usually confers some of its powers on the master (as noted for the imp in the MM). A succubus, for example, may provide her master with invulnerability to surprise and resistance against all types of fire, as well as her usual fiendish immunities (such as half damage from cold and gases, and resistance to electricity and poison). More importantly, fiendish familiars enable their masters to memorize and cast spells as if they were one level higher, provided they are within one mile per level of experience. Finally, the necromancer gains bonus hit points, equal to the maximum hit points of his or her familiar (regardless of proximity).
Of course, should anything tragic ever happen to a fiendish familiar, the penalties are far worse than those suffered when an animal familiar dies. When a fiendish familiar dies (or more appropriately, has been banished from the necromancer's plane of existence), the wizard must roll a save vs. death magic or perish; even if he or she survives, the necromancer loses four levels of experience. These lost levels may be regained normally or by powerful magic (such as wishes or restoration spells). Given the harsh penalties involved, a necromancer will not subject even a fiendish familiar to unnecessary risks.
A more powerful fiendish familiar can be a major nemesis for the party in its own right. In some of Clark Ashton Smith's Hyperborean short stories (such as "The Last Incantation" and the "Death of Malygris"), the necromancer Malygris was served by a fiendish viper of tremendous power, that singlehandedly defeated the most powerful wizards of an entire kingdom when they came to plunder his master's tower. Such a creature was clearly more of a trusted ally and advisor than a pet to the arch-mage, no doubt gated in from the Outer Planes and approached with a suitable bargain to obtain its willing service. Although these allies are not strictly familiars in the sense that they do not share a spiritual bond with the necromancer, they do considerably augment the wizard's powers and knowledge, simply by working with him or her. Clearly, a powerful necromancer may have a formal familiar (mundane or otherwise) and any number of such extraplanar allies who might take on the role of lesser henchmen.
Please keep in mind that the previous discussion of extraplanar familiars has implied that the wizard is completely evil in alignment. No fiend will willingly serve a good master, except perhaps to twist him or her into a servant of evil. Clearly, powerful necromancers of neutral (or good) alignment will attract other kinds of pets, messengers, and allies from the Outer Planes. A pure-hearted Witch may be assisted by an archon or a lesser deva. A powerful Philosopher, on the other hand, may prefer to recruit familiars from the Elemental Planes, summoning an elemental, aerial servant, invisible stalker, or even a genie into service. Necromancers of all alignments can thus gain the powerful benefits provided by an extraplanar familiar.