The character of Lucan's witch Erictho has provoked considerable unease amongst modern commentators. Doubts have often been voiced as to whether she constitutes a fully plausible literary creation and a consistent psychological character. For Ahl the witch is "a creature on the borderline of plausibility."  According to Gordon, "the figure of Erictho refuses to be exorcised," because of the chasm between her vast powers and her insignificant duties.  Johnson, on the other hand suggests that Erictho is the "first recognizably modern witch in European literature." 
Lucan's witch is undeniably a composite figure, developed principally from three existing literary models, the first of which he transforms dramatically, the second he subjects to rhetorical amplification, and the third to pattern negation. As a result, the character of Erictho has three distinct aspects: 1) the Virgilian fury, 2) the Lamia, and 3) the anti-Sibyl.
1) Erictho as Dira
As the analysis of Hardie demonstrates, the character of Erictho is modelled in part on the Virgilian Fury Allecto.  This aspect is more in evidence during the necromantic sequence itself, and is rather less prominent in the earlier narrative. Like Allecto in Aeneid 7, Erictho is shown taking steps to assure that peace will fail, and that the opposing factions will do battle in Thessaly:
Another characteristic reminiscent of the Virgilian Furies is Erictho's abiding thirst for armed slaughter. She is explicitly linked with the civil war, and pointedly said to master and possess the souls of the slain:
Following these broad parallels, the link with Allecto is first made explicit just prior to the necromancy in the description of Erictho's Fury-like attire and her hair bound with vipers:
When subsequently Erictho, annoyed at the delay in the revivification, furiously lashes the recalcitrant corpse with a live snake (verberat immotum vivo serpente cadaver, 6.727) she is mirroring Allecto's treatment of Turnus in the Aeneid :
Moreover, Erictho's prompt abandonment of friendly persuasion after first "praying" politely to the gods, may be modelled on Allecto's assault on Turnus at Aen. 7.415-66. In both cases, the assailants quickly revert to threats and abuse after the failure of a more placid approach. There is also a significant parallel between Allecto's description as rabido ... ore (Aen. 7.451) and Erictho's spumantia... ora (6.719).
As Hardie observes, the symbolic equivalence of Erictho and Allecto also helps to explain why Lucan's witch summons only Tisiphone and Megaera at 6.830. For, speaking intertextually, summoning Allecto would be an incongruity - it would amount to Erictho summoning "herself." 
But if Erictho shares a number of traits with Virgil's most terrible Fury, she nonetheless clearly surpasses her in dreadfulness. This is in part, no doubt, because unlike Allecto, Lucan's witch is self-directed. Rather than meekly doing the bidding of the gods, Erictho has an agenda that is entirely her own. Whereas Allecto stirs up war in Italy at the command of Juno, Erictho promotes war in Thessaly for her own pleasure. Thus, Sextus' request to her pointedly appeals to her own self-interest:
Moreover, Erictho surpasses the power of the Virgilian Furies in that she harries not just ghosts or humans, but the very gods themselves. The fact that she commands such immense power though a mortal (vita, 6.515) is striking. She is in effect an amplified and geographically-inverted Fury: she dwells in the upper world and terrorizes the forces of the nether realm.
2) Erictho as Lamia
The description of Erictho that concludes Lucan's introduction at 6.507-569 stands as one of the most horrific portraits in Latin literature. It presents a truly demonic figure in a rhetorical amplification of a category that is itself already entirely negative, the popular image of the night witch, or Lamia.  As Gordon notes,
Some examples of typical Lamia motifs cited include: solitary squatting in expropriated tombs (6.510-13); death-like pallor and intolerance of the sun (515-20); cannibalism (6.540-43); false appearance (6.564-8).
Erictho, then, is beyond the pale of "ordinary" literary witches. In fact, she is described by the rhetorical technique of "negative enumeration", the contrast being drawn between the usual Thessalian magae and herself.  Although the Thessalian witches are a thoroughly evil lot (Lucan frequently emphasizes their wickedness; e.g. 6.443, 6.509, 6.430-31), the introduction to Erictho makes it clear that her foul acts are without parallel:
The difference between Erictho and other witches is that the latter - in Lucan at any rate - have no evident connection with the underworld. In the lengthy general discussion on Thessalian witches (443-506), their powers are pointedly limited to the upper realms, that is, to the superi, mortal men, and the animal and plant kingdoms. Erictho, however, has an intimate relationship to, and knowledge of, the underworld, of death and the dead:
The emphasis on death and the dead here is quite obvious. As Rosner notes, Lucan's has deliberately made the realm of the dead Erictho's exclusive domain; she alone of the "living" seems to know Hell and its secrets.  The fact that she lives underground and outside the city serves to link her to the dead, who were buried outside the city. Indeed, her very appearance, her deathly pallor, suggest a strong association with death:
The physical description of Erictho is followed by a further elaboration of her foul activities. Once again, Lucan emphasizes her special affinity with death, and the special potency of her methods. Her steps blight fields of crops, and her breath befouls even the purest air (6.521-2). She does not pray to the Gods, but rather constrains them with a single incantation:
As the passage progresses, Erictho's activities become increasingly gruesome and violent. She mutilates corpses (6.540-53, 562-69), and takes a ghoulish pleasure in killing when her grotesque ceremonies require either fresh blood or body parts: hominis mors omnis in usu est (6. 561). As Rosner observes,
3) Erictho as anti-Sibyl
Perhaps most importantly, Lucan's Erictho stands as the antithetical counterpart to Virgil's Sibyl. She thus serves as Sextus Pompey's underworld guide and overseer, just as the Sibyl does for Aeneas in Book 6 of the Aeneid. Erictho's role is to perform the requested necromancy, just as the Sibyl must facilitate Aeneas' nekyia. And indeed, Sextus addresses her as if she were, like the Sibyl, a benefactor of humankind:
As Virgil's Sibyl willingly helps Aeneas in his pious quest, so Erictho is more than ready to assist Sextus in his impious pursuit. Moreover, Lucan's witch, like her Virgilian counterpart, treats her petitioner with considerable deference. Thus, Erictho begins by addressing Sextus with the respectful epic vocative o iuvenis (6.606). This is close in tone to the Sibyl's address to Aeneas at Aen. 6.83: o tandem magnis pelagi defuncte periclis. Furthermore, the witch later describes Sextus as fortis (6.773) in the course of the necromancy. This is again close in tone to the complimentary description of Aeneas by the Sibyl (pietate insignis et armis, Aen. 6.403) during their underworld journey.
Following the precedent of Virgil's Sibyl, part of Erictho's function is explanatory and didactic. She takes on this role with apparent relish in her first words to Sextus, explaining the extent and limits of the power of witchcraft - offering information and services well beyond what her petitioner strictly required:
This speech, a programmatic explanation of the task at hand, serves as a counterpart to the Sibyl's speech to Aeneas at Aen. 6.124-55. Lucan's use of Erictho in this kind of didactic role helps to assimilate the witch to the Virgilian Sibyl. Overall, the allusive strategy involves a subtle dialectic of appropriation and difference which helps to define Erictho against her Virgilian prototype.
There are some striking reversals of the Virgilian model. To begin with, while the Sibyl tells Aeneas what he must do (accipe quae peragenda prius, Aen. 6.136), Erictho is perfectly happy to undertake the entire necromancy herself. Moreover, while the Sibyl warns Aeneas that only the initial descent into Hades is easy (facilis descensus Averno / ... / sed revocare gradum superasque evadere ad auras, / hoc opus, hic labor est, Aen. 6.126-8), Erictho dismisses Sextus' request as altogether trivial. The witch uses the splendidly inappropriate adjective pronum: since in Sextus' case there will be no descent, pronum is a deliberately perverse echo of the Sibyl's phrase facilis descensus Averno.
The vast moral chasm between Erictho and the Sibyl is nicely brought out by Lucan's account of their respective preparations. While the Sibyl piously insists that the unburied corpse of Misenus (exanimum corpus, Aen. 6.149) must be properly buried before Aeneas embarks on his underworld journey, Erictho specifically requires an unburied corpse (described similarly as exanimes artus, 720) for her undertaking. As Masters points out, there is a clear connection between Erictho's cadaver and Virgil's Misenus.  This facilitates one further inversion: whereas the Sibyl's rites begin within a burial, Erictho's conclude with a burial.
Likewise the Sibyl's pious exhortation to Aeneas show courage (Aen. 6.258-60) finds a perverse and impious echo in Erictho's chastisement of Sextus and his men for their cowardice (6.659-66).
Finally, the magical rites described in the Erictho episode contain traces of the Virgilian passage in which the Sibyl sacrifices to the nether deities. Lucan adroitly exploits the intertextual possibilities to underscore the immensely greater power of his witch. Thus, in imitation of Virgil's description of eerie sympathetic noises that follow the Sibyl's sacrifice,
Lucan offers a far more extensive list of fearful elemental sounds emanating from Erictho's own mouth:
Throughout the passage, Lucan carefully reworks the underworld scene in Aeneid 6, elegantly combining two separate Virgilian motifs - the unburied corpse of Misenus and the search for the golden bough (Aen. 6.136-235) - into Erictho's search for a corpse to use for the necromancy.  Instead of Aeneas descending to the underworld to receive a prophecy, Lucan has Erictho summon a ghost to the upper realms. Thus, while Aeneas and the Sibyl are living beings among the dead, in the Bellum Civile it is the prophetic corpse who is away from his proper environment. 
Although Erictho and Sextus do not actually descend to the underworld in the manner of Aeneas, Lucan manages to echo many of the effects from the Virgilian nekyia. Thus, the cavern in which the necromancy is performed suggests a Hell-like scene:
Moreover, like the Virgilian entrance to Avernus, Erictho's cavern is forever shrouded in darkness by thick foliage (arbore opaca, Aen. 6.136; taxus opacat, 6.645). Lucan even leaves a very slight suggestion that Erictho might actually be conducting a veritable nekyia:
Even Lucan's brief conclusion (6.820-30) to the necromantic passage evokes the close of the Virgilian underworld scene: Erictho accompanies Sextus back to his father's camp, just as the Sibyl accompanies Aeneas back to the upper realms.
Other Sections: Sextus Pompey | Cadaver | The Gods