Boldly, You Sought the Power of Ragnaros

What a long, strange trip it's been.

Here's the mount reward: [item=115484].
Until that page can be updated with the correct 3D view, the mount's view in 3D model can be viewed here.

This Achievement can be completed during the 10th Anniversary event for World of Warcraft. To complete it, you must defeat Ragnaros in the boosted 40-man Raid Finder version of Molten Core, that will only be available during the Anniversary Celebration.

This version of Molten Core is for level 100 only, so you'll have to level quickly.

One of WoW’s classic 40-player dungeons, Molten Core, will be available in Raid Finder during the anniversary period for level 100 players. Making it through the underground gauntlet and defeating Ragnaros will reward players with a Core Hound mount and a Warlords of Draenor Raid Finder–quality helm (100% guaranteed!).

The new dates given are:

Start: 2014/11/21
End: 2015/1/06

According to the WoW Events page on the Blizzard site, the Anniversary Celebration Event will begin November the 18th and last until December the 2nd (2014).

And Warlords launches on the 13th of November. Five days (17th/18th) seems like a short amount of time to let people get leveled up and ready for MC, even if it's LFR. Ten days (23rd) is much more reasonable for a chunk of the playerbase, and would give another chunk of players time to level during the event itself and catch the raid before it's gone.

I suppose it would be nice to see a level scaling option for this Event so slower levelers & busy people (isn't this the group for whom LFR was created in the first place?) can still do the raid before the event is over (and Altoholics can bring a few characters, if they want to do so).

The raid will not include a scaling option, sadly, and will only be available at 100, but the Battleground will have two brackets, 90-99, and 100.

It is important to note, though, that you don't have to worry about missing out on (much in the way of) cosmetic gear or stat items by not running your alts through MC. There are now helms. There are now fire-y gem helms for everyone.

[item=118574], the tiny [npc=54255] is obtained thusly:

Repeat MC runners will have a chance to get Hatespark the Tiny and the Flames of Ragnaros enchant illusion :)

Ion Hazzikostas had this to say on twitter:

It's set up as something you'd run once to get your mount and for some throwback fun, not to compete with Draenor content.

Just logging in during this Anniversary Celebration will also reward you with a [item=115301] battle pet.

I ran Molten Core on the second day with a pretty good group. The boss guides on the 10th anniversary are pretty spot on, but here are a few pointers on general strategy and some specific bosses.

1- Duration

Expect 2+ hours. It took us almost 2 and a half hours and we had a couple wipes. There is a ton of trash mobs and you need to clean about 90% of them. This is not your regular LFR segment. They wanted to recreate the experience of the vanilla raid and obviously were not really concerned about the amount of time it takes. I can't tell if the experience matches the original but I know that it took 3 hours counting the queue. You've been warned: don't queue up unless you have a lot of time ahead of you.

2- Boss Fights

The mechanics are very simple compared to a modern raid. They are even simpler than a modern dungeon boss. This makes the process a little tedious.
Also, no modern reset for the bosses: they will follow you to the entrance of the raid if you attempt to flee! In a few words, if you die, wait until it is actually safe to release.
Your major cooldowns (bloodlust, 10-minute dps cds and such) do not reset after a boss fight. It's not really an issue unless you wipe though. It takes some time to go from boss to boss.

3- Trash mobs

Do not overpull. It's easy to get overwhelmed even with 2 packs of trash mobs. Better safe than sorry. It's a long walk if you happen to wipe near Sulfuron.

4- Specific Boss Fights

Lucifron: Be really careful not to pull him with a pack of hounds, or you'll wipe. Apart from that stick with the Wowhead strategy.

Garr: there will be some movement, so clean the room thoroughly before engaging him.

Baron Geddon: the only one with a specific mechanic that you need to really watch for as a dps. Please don't explode in the middle of the raid, that's rude.

Sulfuron Harbinger: We marked the four adds and assigned "interrupt groups" to make sure every add would get some love.

Ragnaros: He submerged at 4% health and the tanks properly rounded up the sons of flame, no problem there. Since that was on the second day of the event, you can be almost certain that next month, with the increase in item level, he won't even have a chance to submerge at all.
Oh, and don't forget to loot him. The crown is on his corp...err Hammer. So is Hatespark and more than likely the enchant transmog.

Good luck and have fun!