So, there is difference between Soul of Iron, Hardcore Challenge, and Road to Ragnaros.
Hardcore Challenge
This has been a staple of the HC community for quite a while. It’s basically a solo-self mode. You’re not allowed to group, except for dungeons - of which you’re allowed 1 run each, you’re not allowed to trade, and you’re not allowed to use outside resources (sending yourself things in the mail), and you’re not allowed to die. If you die, you delete. Before the addon was developed, every run had to be fully recorded from start to finish, either through your computer or using vods on twitch. These recordings then had to be uploaded, watched, and verified by group of mods within the HC community. If you passed, your name went into the Hall of Legends. You can find more information about it at classichc dot net. At level 60, you become Immortal and are free to do whatever you wish with your character, including dying.
Soul of Iron
This was implemented by Blizzard as a nod to the hardcore community. There were no rules, no “get out of jail free cards” for DCs or griefing (like being mind controlled, flagged, and then killed). There are no rules to this. This is basically a player deciding on their own that they want to play without dying. They can group, use the AH, do whatever they want basically.
Road to Ragnaros
A community event based off the Hardcore event where the community tries to kill Ragnaros without dying. To participate in the community event, you must have completed the Hardcore challenge, following all the rules, and have your 1-60 run verified. Once you reach 60, instead of becoming Immortal, you now enlist in the Road to Ragnaros. You can trade, group, and do dungeons freely, but only with verified HC level 60s who have enlisted in R2R.
The reason why this blew up so much is because of the addon that was developed. Before, every second had to be recorded then watched and verified. With the addon, it tracks everything. Whether you’ve grouped, dungeon IDs, blocks mailboxes, announces deaths - basically does what before someone had to spend hours doing. Not too mention, a lot of big name streamers got onboard for the Road to Ragnaros event, leading to an explosion of people all wanting to play HC, like their favorite streamer and his closest 2000 friends.