I've started a new party with Cultists faction. During of initial scouting I've managed to pacify a region with Kazanji faction. But before my army returned there, this region was colonized by Broken Lords. Anyway I've decided to convert nearest village to gain additional unit and promote my faction quest. Few turns later Broken Lords destroyed this (defenceless!) village. Surprisingly, I've received achievement for it. More important that this is achievement from the Tempest DLC which I even don't own:

It Came from the Depths
Attack an army containing only land units with an army containing only submersible units.

You can see it in my Steam profile.

Two savegames, which can help you to trace this bug:

1) turn, when this achievement was received

2) previous turn