A 20 man mythic team went in on Monday and downed the first encounter. Two team members had seen the fight before in other groups, but this was our first Hellfire Mythic run as a guild. The winning attempt looked great until the very end when things suddenly went completely off the rails. As pandemonium reigned, the cannons got off the final shots and we beat the encounter.
We are actually the first guild on Sisters of Elune/Cenarion Circle to register a Mythic kill. I imagine that other guilds have downed some of the fights, but you need to have 16 guild members in the 20 man raid to get guild credit. Even combined, our two realms are small. This has made it difficult for guilds to field a full 20 man team. Our successful team had 18 guild members as well as two folks from Rusty Blades - both of whom have been an integral part of the team for most of this expansion.