theHunter: Call of the Wild™

To get Halloween achievements, you can download the old version of the game using DepotDownloader, with the following code:
dotnet DepotDownloader.dll -app 518790 -depot 518791 -manifest 571650642277926356 -username **** -password ****

More details about using this program in this guide

This is only achievable by down patching the game to an older version.

A fairly involved working solution was discovered here by steam user NarTeKK:
https://steamcommunity.com/app/518790/discussions/4/3826426358094154079/#c4138313123977942710

This one is most likely bugged as it unlocks only when you don't use taxidermize option. I've harvested all 4 diamonds and taxidermized them, so it did not pop at first, but then I just rolled back my population file and harvested one more diamond again without taxidermizing it and achievement finally popped.