Dark acolyte warhammer fantasy battle 8th edition vampire counts vampiric power
This vampiric power has undergone a complete transformation from in 7th edition where it granted the vampire with it an additional magic level. This had now been added to the normal upgrades for both vampires and vampire lords in 8th edition allowing hero level vampires to be either level 1 or 2 wizards and vampire lords to be anywhere between level 1 and level 4 wizards.
In 8th edition the dark acolyte vampiric power now allows the vampire with it to add an extra d3 to the casting value whenever they successfully cast invocation of nehek. Note that it only adds to the casting value after the spell has been cast, this basically means that dark acolyte makes invocation of nehek harder to dispel for your opponent.
Out of the special characters both versions of Mannfred von Carstein, Count Mannfred and Mannfred the Acolyte have the dark acolyte special power and using either of these can be a good time to try out the dark acolyte vampiric power.
Pros and cons
Pros for dark acolyte are that it forces your opponent to potentially use more dispel dice to stop your invocations of nehek, especially if you have several vampires with this vampiric power. It can also be combined with the blasphemous tome upgrade on a mortis engine. This will further power up your spells, this combination adds up to an extra 5 to the casting value of any vampire casting invocation of nehek. If several vampires are using dark acolyte within range of a mortis engine this can make repeated castings of invocation of nehek difficult for your opponent to dispel.
Cons are that as it only effects a specific spell, invocation of nehek and each wizard with the spell may now only cast it once meaning that you can only get limited use out of the ability. Even hero level vampires are fearsome in close combat now and will often benefit more from combat related vampiric powers or one that benefits the units they are accompanying. Also to get maximum effect out of dark acolyte you really must take several vampires with it.