The Chosen

This achievement, and it's corresponding title, [title=493], are no longer obtainable.
They were awarded for completing the Trial of Valor raid on Mythic Difficulty during the Legion expansion, without dying once in a single run full-clear. It was tracked per-person, so if individual members died but others didn't, the living members would still get the achievement.

This achievement was moved to a Feat of Strength and made unobtainable upon the release of Patch 8.0, the Battle for Azeroth pre-patch, released July 17th, 2018.

In addition to the title, this achievement also awarded the ensembles for the Mythic Trial of Valor transmogs, via [item=143617], which was mailed to the character on which this achievement was completed. This was and is the only way to obtain the Helmets, Shoulders, and Cloaks of the sets. However, there are similar cloaks with the same texture but different shapes available from Mythic ToV.
(The original confirmation for this was according to this image gallery by the first guild to complete this achievement: http://imgur.com/a/kQ11W)

The [item=143617] was BoE until June 15th, 2018, where it was changed to be BoP, to avoid it being sold to other players or being sent to alts where it could have been opened for ensembles of armor types for classes other than the one that the achievement was completed on. With it being made unobtainable on July 17th, it was also changed to be a grey item that cannot be opened, making the armor pieces exclusive to the ensemble completely unobtainable.

I can confirm that even if the raid wipes but you happen to stay alive through vanish,invis, or any other means, you don't lose your buff. We wiped on Helya but I was able to vanish and the boss reset. We killed it on the next pull and I was the only one in my raid to get the achievement.

Looks like this is not "that hard". Apparently this is a personal achievement. You get the buff as soon as Odyns start his roleplay (before he's pulled). You need to be inside the instance or you will miss it. After that you need to "just" oneshot all the bosses without diying. As I said the buff is personal so it doesn't matter if anyone else die but you can't wipe, and need to kill all 3 bosses first try. Maybe you can avoid it if you use feint death or vanish or some mechanic that prevent you from diying if your raid wipes.

Edit: Looks like this was "hotfixed". Now if you lose the roleplay but you are inside when Odyn is pulled you will get your buff applied to you as soon as you pull. This is nice for people who join late and got summoned just before the pull.