Not sure, probably in a totem…
Funny thing is I had one drop like, yesterday.
It’s not the first time someone mentions this item in particular… I never imagined this thing being a hard farm.
Just keep farming I guess? 
Sorry I can’t be more helpful here.
Mythical Blightshard amulet is extremely friggin’ rare.
Almost to the point where I’d file it as a bug.
It took me ~400h to find one, after I started to farm for it.
No exaggeration, I made my Ghol Ritualist on the test patch for 1.2, in August or September of '23.
It dropped a year later, from a totem iirc.
Gotta say, i think some items are linked to the “tribe” they are connected to (like Aether epics/legendaries dropping only from Aether totems). I’m trying to farm it 'cuz i’m trying to build a Death Gates Ghol Ritualist but with skeletons too, i love having lots of pets (possibly with high health regen, my Lost Souls Cabalist is more than enough for superbosses)
The items linked to totems are just 11 Legendaries, which were introduced with those totems.
Those items can also drop anywhere, just have a higher chance from totems.
Other than that, there are some MIs and boss-only epics and legendaries, those are bound to their specific boss. Blightshard is not one of those.
It’s just really rare, for whatever reason.
I see, thanks mate. Now idk if i wanna finish my ritualist (death gates, like maya’s latest build) or build a cabalist (classic lost souls vitality). Mogdrogen pact vs blood of dregg, what to do…
We should probably ask @Maya for that. I may have made this build here, but when it comes to pets I barely know what I’m doing. 
Oh god no i asked him too much already XD.
Seriously tho i feel like manipulation makes the difference (provides a ton of total speed to pets) and blood of dregg has an instant heal other than the basic health regeneration, on the other hand with the ritualist build you dont care how many skeletons die since the cowl lets you summon seven for free each time
I haz been summoned
For me on my Skeleton Ritualist the choice came down to the extra Vitality RR provided by Devouring Swarm being greater than Vulnerability’s. I made Death’s Gate Cabalist too, but didn’t really post it due to its performance being inferior at the time.
You can do fine with Cabalist as well, especially with Master of Death not being as bad anymore.
(The heal vs regen is moot for Skeletons since they dont really survive long enough for that to matter)