Runemals

The Last History chapter can be found at the dark blue flag which is on the ship in the lower right of the map. You need to have finished the swamp history chapter (first quest at the purple flag) before this one unlocks. Before you are eligible to play this you need to have one or more Runemals at third evolution. Once you meet the requirements go to the dark blue flag on the ship on the bottom right side of the map and win the battle.

This achievement will unlock whether you win or lose this quest. In fact, on their forums, the game’s developer says that it is impossible to defeat the final boss though I have heard some people claim that they were able to defeat it.