This pertains to destroying the Mongols camps, outposts and strongholds.
There is a certain element of exploration needed for these, happily, whilst exploring, if you missed one, you can sometimes find a random civilian in friendly zones that when spoken to, will point you towards these areas.
This can be done before recruiting all your allies during Act 1.
Some of these are also behind main missions and side stories, so complete as many of thses as possible.
An extended guide can be foubd here: