Challenge Accepted Achievement
After obtaining NightSpawn, have him kill 100 enemies to unlock his quest (He will challenge you to a battle after he has killed the 100th enemy). Upon beating him you will unlock this achievement and most likely obtain the "It Never Hurts to Help!" achievement for completing all Puppet Quests as this is the most obscure unlock condition among them.
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2857218208