Necromantic spells are used slightly differently to other spells. Normally a spell can only be cast once per magic phase. This is also true of Necromantic spells when cast by most wizards. However, in the case of Undead wizards (Liche, Vampire or Necromancer) the spells work slightly differently.

When used by an Undead wizard, a Necromantic spell can be cast over and over again during the same magic phase. If an Undead wizard has sufficient power cards left he can continue to cast the same Necromantic spell. The only restriction is that it is impossible to successfully cast a Vanhels Danse Macabre spell on the same unit or target more than once during the same magic phase.

The spell could be recast onto the same target if it is initially dispelled. Note that the same restriction also applies to the Staff of Damnation. This staff effectively casts a Vanhels Danse Macabre over every unit within 36". A Vanhels Danse Macabre spell will not affect a unit already affected by a Staff of Damnation and visa versa.

The following is an excerpt from pages 28 of the Warhammer Arcane Magic supplement in the Necromantic Magic section.

An Undead wizard may cast the same Necromantic spell more than once during a magic phase, so long as he has sufficient power to do so. Roll a D6 after the spell has been cast and consult the table below. You must roll each time the spell is cast. If the dice roll is less than the number shown on the table the spell may not be used again that magic phase. If the roll equals or exceeds the number shown the spell may be recast.

Liches and Necromancer Lords do not need to roll if they want to recast a Necromantic spell.