They obviously don't ride horses and have the associated saving throw modifier but nonetheless, in a protracted combat, point for point, they are probably more effective than Black Knights.
The reason I say this is because Grave Guard are cheap for what should be considered an elite unit. Also, they can be equipped with great weapons and, if used smartly, will often strike first in all rounds of combat, including the 1st round even if they don't charge into combat.
Giving the Grave Guard a great weapon to replace their default shield for 1 measly extra point is an option you should probably always take if you are using one or more Corpse carts and supporting Necromantic magic. This makes Grave Guard on a par with, let's say, White Lions and yet Grave Guard will only cost you 13 points while a White Lion will cost you 15 points.
Add to this the Wight Blade special weapon characteristic and the fact they will never rout or respond to psychology (unless they're unfortunate enough to meet Harry the Hammer) and you are bagging yourself quite a bargain by taking the Grave Guard into battle.
These are a nice addition to both Grave Guard and Black Knights. They're not brilliantly effective, they just confer the killing blow ability to the wielder. *
* I prefer to call it the killing wound ability to remind myself that it kills on a roll to wound of 6 (rather than a roll to hit of 6)
The killing blow should bring you an extra kill every couple of combat rounds against nearly all enemies. What is great about killing blow with always strikes first ** is the psychological effect it has on your enemy. It can be a devastating blow for a commander to lose a hero or even a Lord to a lucky random blow from a rusty Grave Guard weapon. This could be enough to disuade the enemy from fighting your Grave Guard with his characters - especially as he knows they will fight to the very last dusty breath.
** Take spellcasters with the Master of the Black Arts vampiric power and a Corpse Cart or 2 within 6 inches of the Grave Guard. This way you should be able to cast the Corpse Cart bound spell or the Necromantic Vanhels Danse spell enough to ensure your Grave Guard benefit from the always strikes first rule.
In Summary, make sure you always include a Grave Guard unit in your army and use them to take on the enemy's best. Give them some vampiric support and a magic banner and you will not be disappointed in their performance. They are some of the best value for points troops in the whole Warhammer Fantasy Battle World in my opinion.
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