Update: It seems I was wrong and Legendaries WILL in fact be working in Mythic raids right from the start and will be affecting the progress race. It seems Blizzard decided not to do the right thing as Watcher said they would, and will be relying on the 1 Legendary per character limit to not make them huge game changers (the Class Order Hall passive will not be ready by the time Mythic starts so it is only one). Well, let's hope the race doesn't end up being about who got the exact Legendaries they wanted then...
In a break from the grueling Mythic split dungeons all top raiders are doing at the moment, Sco got quite excited to see his first Legendary drop, and apparently it was one of the better Warrior ones as well:
Thankfully, these very powerful items will not be allowed in Mythic raids (at least at the start), so they won't be ruining the progress race with additional randomness. Also thankfully, it means players in top guilds will only be spending incredible amounts of time gearing up, instead of insanely incredible amounts, so there's that. Keep in mind the latest we've heard on the Legendaries not being useable in Mythic raids is from Watcher in February, so let's hope it is still the case. But enough about that, on to the actual Legendary in question: