To add flavor to a game an interesting rule twist for the undead is to give them a slight positive modifier when within twelve inches of a graveyard, hunted ruin, or within four inches of a necromancer. A slight negative modifier is applied when within twelve inches of a church, holy shrine, or within four inches of an enemy priest/cleric. All effects are accumulative. Of course when one positive and one negative both apply they cancel each other out. Example : Undead unit is fighting need a holy shrine but is within four inches of a necromancer. What type of modifier you choose depends on your game system and how much you effect you think the rule should have on undead units. A simple idea would be to add one attack dice for a positive each effect and take one away for each negative.