During my first game, I attacked with 8 undead units. The necromancer was able to create 2 reanimates, giving me 10 units. I then won the battle without losing any of my units. Because a hex can only contain 8 units, what happens to the 2 additional units? Are they destroyed?

I think they're forced to retreat and destroyed if unable to do so. I could be wrong though, I don't have time to check the rules right now.

Looking for something from the rulebook, a page, perhaps?

P.18 Movement restrictions (paragraph 8)

"The only time a player can exceed this number is when he is starting a battle in an enemy or neutral area, in which case he may bring any number of units into the battle. If he wins the battle, any excess units must retreat to a single area (see “Retreats” on page 22)."

As XAos and others have said, if the ATTACKER has too many units after the battle, the excess units retreat (if possible), and are destroyed if they cannot.

It's also possible for the DEFENDER to have too many units (really only possible with the Necromancer ability); in that case, the extra units are simply destroyed AFTER the battle. Corey has clarified they can exceed the 8-unit limit during the battle itself, though.

Now the next question would be if you can retreat more than just the excess to avoid destroying units during winter.

For example, since I had only a food level of 3 and nine units in a hex, I would retreat 3 so that I would only lose three units come winter instead of retreating only one and losing five.

You may only retreat the excess.

Are the retreated units routed?

GooberVA said:

Yes, retreated units are always routed. The reason units rout after retreating is to ensure each unit only gets to move once per year, even if it ends up retreating to an unactivated hex. There are, of course, exceptions, but those are exceptions and not the main rule.