I just started playing on Hardcore realm, and got quite confused by the graphical effect where character model turns gray, lift cloud of dust and purple orb spiral around.
From what I could find on the internet, the graphical effect is related to Soul of Iron, what I couldn’t find out is what exactly triggers the graphical effect.
Sometimes it does not repeat for quite some time, sometimes it’s being shown twice within one minute.
Does it indicate some successfully completed challenge of other players or something?
It appears to me as if the frequency of this graphical effect is just random.
The orb demonstrates the effectiveness of BlizzRNG™ 
This buff and effect showed that character has never died before in Season of Mastery (16 november 2021 - February 2023) unlike on Hardcore realms, characters having this buff played along common characters on SoM realms.
Blizzard reused this effect on Hardcore realms to distinguish others HC Self Found characters apart from regular HC characters that could use mail, trade with each other and use AH as some reference to unofficial Ironman challenge.
Nope, as a normal HC on HC realm you go to IF and speak to whatsisname (Watcher Fero?) and have buff.
Strange have a couple of new SF and couple on none SF HC characters and only SF ones got it.
yeah thats because SF-Chars will get the buff right on start while HC cars have to opt-in to the buff.
Strange approach, but OK.