You need to reach at least rot level 5 then either collect 4 spirit stones or the Wyld Talisman (thanks StK I). As soon as you have both, you should instantly unlock this achievement (and maybe die?
https://www.reddit.com/r/armello/comments/457yrh/i_got_drop_...). To reach rot level 5, use rot items or get killed by banes. If you've already completed the rot victory, use the Spoil amulet since it lets you start with rot level 2.
To collect the spirit stones, find them in dungeons, grab them from sanctuaries, complete quests, or use the one spell card that summons one. It'd be best to get three of the stones before reaching rot level 5 since entering a stone circle (where they appear) while corrupted will kill you.
The Wyld Talisman is a unique (rare) treasure item that sets your rot level to 0 while equipped. As a treasure, you might find it in dungeons or get it from quests though you'll need luck to actually get it. Playing as Amber might help.
Old Solution:
My old method was to collect all 4 spirit stones first then get to rot level 5. I thought I earned the achievement this way which was convenient since it let me enter sanctuaries to get more stones. However, this does not appear to work anymore (thanks LucaBS1980IT). The game has been patched since I first earned it.