Necromantic teams are a variation on the theme of an Undead team. They are one of three undead type teams currently legal for play in Blood Bowl - the other two teams being Khemri and the Undead. (Vampires are not a predominantly undead team, and so aren't generally classified as such, despite some similarities.)
Necromantic teams differ from the other two teams predominantly in terms of their options available, most notably due to the inclusion of Necromantic Werewolves. The team is generally regarded as the fastest of the undead type teams, and the most capable of multiple touchdown victories.
0-2 Flesh Golems
Play Style and Tactics
Typical play of a Necromantic team revolves around utilizing the speed of the Ghouls and the blitzing power of the Necromantic Werewolves to conduct a fast, aggressive running game. The team lacks some of the hitting power of other Undead teams, and as a result, needs to rely upon maneuver as much as injury. While the Werewolves are very handy players, as are Flesh Golems, Necromantic teams tend to be less resilient than their Khemri or Undead counterparts. That said, the team can be incredibly effective, particularly upon skill increases over the course of normal league play.
The Necromantic team is best characterized as a Hybrid style team.
Famous Necromantic Teams
Two of the earliest referenced Necromantic teams appear in 2nd edition. These are the Bruendar Grimjacks and the Westside Werewolves.
The best option for building a Necromantic team is probably to consider the use of alternative miniatures, though most of the miniatures can be found in "official" form from Games Workshop. There are numerous options available from a wide variety of manufacturers, especially when it comes to models like Werewolves. One good source to begin your search is the Fantasy Football Store. Gaspez Arts and Greebo both carry miniatures that are specifically designed to be used by Necromantic teams. Impact's range of Deadlings is also quite helpful in this regard, and will mix well with the Greebo Halloweenies.