I've been praticing a lot a list who use the Lords of sacrament Battalion recently, because I want to use it in a tournament in 2 weeks. If you are playing with realm rules (my next tournament will use them), the battalion is awesome! For 130pts, I get 3 more cast per turn, 1 more CP (I'm regularly using 3-4 CP on Arkhan's command ability on the first 2 turns so this is a boon), 1 more artefact and lower drop count for my list!
With all the spells available with the realms this gives me a lot of options and I really fell like an all powered evil wizard during my hero phase! (Arkhan with +4 to cast really help this feeling! ) Last game I played, we were on the realm of Fire and I boosted 20 Grimghast with Van Hels, Inferno blades (+1 dmg) and stoke rage (+1 wound and charge) and I can assure you this was a nightmare for my Stormcast Opponent! They destroyed 3 vanguard palladors, 5 evocators and put a few wounds on a lord celestant on dracoth in one combat phase! Also in addition to these 3 spells on the grimghast, I was able to cast prismatic palisade to protect myself from his range units, cast incadescent form (-1 to hit) on Arkhan and cast curse of years on a liberator unit! If I wanted to do the same thing it would have costed me 3 more necromancers (200 more points if we remove the battalion cost)
The biggest problem of the battalion is that everything has to stay close form one another. So you have to play it like a big deathstar! Throw it in the middle of the board and start bombarding key ennemy units with spells! when you know you have the advantages, you can start splitting the necros from the mortis to help cap objectives. (around turn 3) My turn 1 and 2 usually looks like that :
turn 1
use wolves as screens or flank objectives holder / use Arkhan's command ability 2 times / throw seven spells in the ennemy face / move the blob near the center (2x 40 skeletons are protecting the battalion)
Turn 2
use Arkhan's command ability 1 time / throw another set of 7 spells (mix of offensive/defensive one depending of the situation/realm) / start to expand board coverage with skelies / grimghast are my heavy hitting troops so they are preparing for a good charge agaisnt a juicy target (with buff if possible)
So In conclusion, if you are playing with the realms spells, the battalion is really worth it and really fun to play. Without all the extra spells, the battalion is a lot less strong and I think taking another vampire lord/necro could be a better choice. It also depends on how many spells/endless spells you want to cast each turn.