The night shroud is magic armour, but it is not light or heavy armour, and thus may be worn in addition to mundane suits of armour, like the dragonhelm. It provides -1 AS, so a necromancer with the shroud has a 6+ save, while a vampire lord with nightshroud, heavy armor and shield, riding a zombie dragon, would have a 2+ save.
I'm not a big fan of the shroud on a dragon, though, as it doesn't help against against cannons, nor do its penalties apply to models that aren't in base contact. With the dragon's huge base, that can be a lot of supporting attacks.
I'm not a big fan of the shroud on a dragon, though, as it doesn't help against against cannons, nor do its penalties apply to models that aren't in base contact. With the dragon's huge base, that can be a lot of supporting attacks.