The Banner Saga 2

In Chapter 13, shortly after reaching the Builder's Godstone, there is an event where Dytch will mention seeing some banners from Einartoft up ahead among the dredge. At this point, Krumr will ask to go and check it out. You will have the option to go and investigate with him or refused to look (resulting in forfeiting the achievement and losing Krumr). After a battle you will recruit a number of Varl, the number seems to be random or determined by different factors (I got 27, putting my total varl up to 30 for Oldford, but I've heard of as few as 21).

Later on in the chapter, you will reach the town of Oldford, and should have the required number of Varl for the achievement. The achievement didn't pop for me right away; however, just before reaching Oldford, there is an event with Krumr where he will want to go disrupt an ambush, send fighters with him and he will return just before the battle for Oldford. That is when the achievement popped for me.

Another note, I am fairly certain you can still recruit the Varl without Krumr in your party, but I have only ever played with him in the party so if someone else could confirm or deny that it would be appreciated.