Standard Mage Hero, has an area of effect fire attack, can use scrolls and staves, can also use orbs. Lich Queen is the only undead unit that has an attack that is not "Death" or "Weapon" source.
WARNING!!! CAMPAIGN SPOILER ALERT BELOW!!!
It is highly advised to take the Lich Queen as your primary hero in the Undead Campaign of Disciples 2, primarily because there will be stages later in the campaign where you will be fighting other undeads, even facing those that are immune to both Death attacks and Weapon attacks. Because of this you could potentially end in fighting situations where both armies are completely immune to everything the other army can do since the Undead units DO NOT have ANY other source of damage besides death and weapon, with the exeption of the Lich Queen who deals fire damage.
For this reason the Lich Queen is the best hero to level during the Undead campaign because she can easily get past those situations where the other two heroes and every other undead unit would be incapable and you would have to kill enemy parties by using items in combat and spells on the map lenghtening these stages with several days while you accumulate enough resources.