There's a shield that gives +40 defense and can shield the user for 100,000 damage. There's an uber dagger for rogues, and an awesome bow for hunters. There's even a staff that not only increases both spell damage and healing by 325, but also adds over 1000 attack power to Cat, Bear, Dire Bear, and Moonkin forms.
Nihilum is so far the only group to take on Kael'thas (and let us know about it), but they haven't spilled too much information about exactly how and why these weapons are used. We do know they're meant only for the encounter, and that when you die, the weapons disappear. But how and why they're obtained isn't quite clear yet-- I've heard the weapons are given to the raid when Kael's advisors are killed, but does that mean these weapons replace the raid's weapons or do they have to be looted and equipped? And what exactly is Kael doing during all of this that requires so much crazy power?
As of this writing, Nihilum hasn't yet downed Kael, so they're probably more concerned with going after him then telling us about it. But once they down him (I predicted this past weekend, but we haven't seen anything yet), maybe we'll get some clearer answers about what the encounter is actually like.