Next big test on the raid list after taking down heroic Halfus is the heroic multi-face boss (commonly referred to as Chimaeron) in Blackwing Descent.
No, Chimaeron isn't the dragons that are pictured above (although they're all in the same place). I haven't been able to get him down yet, since he is one tough dude. The reason why I've found Chimaeron more challenging than Halfus is because the Halfus encounter is over in a minute, if your raid can survive the initial rush. In Chimaeron, the healers are working overtime. We've been able to consistently get him down to the final phase. I imagine we're not the only ones who are still struggling with him (or it).