The heroic version of Halion, the final boss of the Ruby Sanctum is now being tested on the PTR and this one is a doozy. Thusfar noone has managed to take down this boss and it looks like it may remain that way until release.
As always, you can never judge an encounter by its state on the PTR as Blizzard habitually revamps content in-house before releasing it to the live servers. However, those changes are typically to make the encounters more difficult. The PTR encounters of Ulduar, ToC, and most of ICC were substantially easier than the versions that went live.
Halion will drop ilvl 284 loot on 25 player heroic and ilvl 271 loot on 10 player heroic which is comparable only to the loot table of The Lich King himself. Therefore it's safe to assume that Blizzard intends for this encounter to be comparable in difficulty to The Lich King encounter. Ruby Sanctum however will not benefit from the ICC raid-wide buff, which may make this encounter effectively the most difficult in the game when it goes live.
There is a good deal of information available on the fight already, and more is forthcoming. The heroic version is not significantly different mechanically, but it certainly looks brutal. There's a new video of the EU guild Affenjungs INC (EU-Frostwolf) attempting the 25 player version. After 5 minutes they barely scratch 70% of his hp. Take a look:
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