Curious Expedition 2

Missable but you will likely be doing multiple campaign runs so can be reattempted.

During the final expedition for Act II, you will be searching for and be told that a Salamander Prince might know their whereabouts. In the second Island (Caverns) of the expedition, locate and visit the Mole village and ask the leader to recruit. One of the possible recruits will be the Salamander Prince, who you can pay for release.

Alternatively you can attempt to rescue the Prince covertly by passing a difficult green dice roll.

Once the Salamander Prince is in your party, the achievement will unlock. You do not have to keep them in your party if you don't want to.