Unicorns: In the first (elf) chapter, right after you speak to the druid that is at the bottom of their big tree, there is an elf to the druid's right that says it's looking for unicorns. Beat him and you'll gain access to the spot that sells them.
Sword Masters (I forget the official name but I think this is close): In Godric's chapter, in front of Count Carlyle's manor, there is a guy in front of a statue who tells you he has been tasked with guarding it by the Count. Beat him in a fight to gain access to the store with the sword masters.
I just started Fiona's section so that's all I have for now. --- Happiness in intelligent people is the rarest thing I know. -Hemmingway
Should probably mention: Other than those two so far every other unit has either been explicitly given to me as part of the story line or I found the store that sells them on the map as I explored, without having to beat any special fights. --- Happiness in intelligent people is the rarest thing I know. -Hemmingway
Those are considered secret because they aren't unlocked for multiplayer when you first begin the game. You have to buy them in single player first to unlock them in multiplayer.
***minor spoiler (wraiths location)***
The wraiths for Necropolis is all the way east, at the east most map (one after the great bone dragon talon), and if you go southeast, you can fight a necromancer to gain access to the cave. This game has the bounty Urolox and bone dragon as well. 2 birds in one stone, I say go for it.