The boss can also be a Death Rite Bird, killing it got me the achievement at least

Should be noted that the "Oldest Jeol" Is the one that houses the dual Godskin Apostle fight. Can't just be any Jeol, but my group couldn't discern which one would spawn them to intentionally achievement hunt this one it just kind of happened when we opened one.

I opened a lot of them but didn't get the achievement

awesome