I have an idea!
Wouldnt be interesting, if undeads werent playing faction, that would be made, as mongols in total war, as invasion ..... there would be one lich/xx behind and would spread poison which would return some units to life when they die as undeads under his command. Also they will be lurking some undead groups/armies around and attacking stuff. So the best way to defeat invasion is to find lich/xx and kill him. To do so, the best way would be that all players would work together or they will fail or make even more undeads. Imagine capital city being sacked which would increase amount of undeads =). But if they fail the army of undeads leave and take x% of your goods (because if all ppl fail the undeads would be there forever killing everyone which would kill the gameplay).
Picture made by Aametherar the orc
Glory to the elves!