I was needing this today, so i did it. Here it is:
AURA: 10 all; Aura of Hate; IF: TYPE(undead,fiend); DMG: 5
Checks in a 10 feet area, all factions, IF the crature is an undead it adds 5 damage (the charisma of the paladin) - I'm using Advanced effects, but if you are using normal power effects. This might work:
AURA: 10 all; Aura of Hate; IF: TYPE(undead,fiend); DMG: [CHA]

