Weakness To Attacktype
All undead units, and only undead units, have a weakness to HOLY. This weakness cause a -50% Damage Resistance penalty to the combined (averaged) resistance of undead units against any attack with a HOLY type. Example: ignoring HOLY resistance boosts, wraiths have a (0 - 50 = -50)% resist against pure HOLY attacks, but a ((0 + 40)/2 - 50 = -30)% resist against HOLY RANGED attacks.
In addition, a number of units are vulnerable to elemental attacks, specifically fire, cold, and lightning. Any attack including one of these types will do double damage to any unit with the matching weakness.