Necropolis Knights divide the opinions amongst the Tomb King players: some won't touch them with a foot long stick and others won't go to battle without them.
They are costly, almost 20 percent more than a Kroxigor, and if given the Entombed Beneath the Sands it will raise the cost with additional 5 points per model (and this bumps up the price so high, that I seriously doubt your opponent will give them the EBtS ability, as the risk of losing such a pricey unit is too high). But they have the best armor save in the whole TK army, a whopping 3+, and if a hero character called Necrotect is within 12", they also get a 6+ regeneration. A special character called Ramhotep can give one random unit a rerollable AS, so if your opponent uses him, the snakes are his only Animated Constructs, as he wants to have that reroll on those snakes. This means no Stalkers, Sphinxes, Scorpions, Colossi or Hierotitan.
They have a toughness of only 4, though, but still have one of the best staying powers in the whole army, especially in the group of 9. The snakes have S5 and poison, and riders have S4 and spears (so S5 on charge) together with Killing Blow. They have plenty of attacks too, and are nice target for the Smiting spell granting an extra attack to both the rider and the snake. If I remember correctly, as per Monstrous Cavalry rules, only the riders can give supporting attacks, so the extra ranks are there mainly to increase their staying power.
9 Necropolis Knight have a huge footprint and they are not the fastest things around (M7), so you should be able to outmanoeuvre them and get flank and rear charges charges, especially if you have lots of obstacles in play. A huge block of Skroxigors could be an effective tarpit. Comet of Casandora is effective, as they are not likely to completely avoid it and don't have ward save (except from a spell of Nehekharan lore). Also they are rather easy targets to Purple Sun and Pit of Shades, as they have an average initiative. Dwellers hurt them too.
Hope this helps.
