HUMANKIND

To get this achievement, you need to direct a military unit (or a stack of military units, preferably) to a holy site. Then, you need to use the Ransack option in their actions, and you need to leave the unit on that tile until the ransack is complete (leaving only Ruins).

The conditions for this achievement appear to be more complicated than the description seems. Sites seem to cease to be holy if you ransack a Holy Site in a secular or atheist state, and I couldn't get it to pop when ransacking a Cultural Wonder that double-counts as a Holy Site. Destroying a Holy Site by using a nuclear missile also does not appear to count; the tile has to transition from Holy Site to Ruins, and not to Wasteland.

So, to be safe, play a run where you plan on making lots of military units, but DON'T enact the civic that makes you secular/atheist. You can ransack one of your own Obelisks of the Gods in the late game when you are mopping up your run's achievements. (You can reload later if you want to undo that.)