First off, the Siege doesn't actually begin in Orgrimmar. Looking at the zone page, we see the following bosses:
- Immerseus: Tears of the Vale
- The Fallen Protectors
- The Sha of Pride
As near as I can tell these four bosses seem involved with the corruption of the Vale of Eternal Blossoms by Garrosh's reckless digging -- Norushen is visible at the end of the Dark Heart of Pandaria scenario, and the Fallen Protectors link to this journal entry
, where we can see that they are members of the Golden Lotus no doubt succumbing to Sha corruption. Most likely, it's related to the chest Norushen was watching that Garrosh's men discovered in Dark Heart of Pandaria. It seems likely that players first have to battle their way through the Vale to confront the Sha of Pride himself, as sort of a mini end boss for the first section of the raid set not in Orgrimmar, but in the Vale.
Following that, we seem to be on track to head into Orgrimmar itself, as seen in the following list of bosses. Only a few of them have journal entries, which we'll link to and discuss again after the link.
- Galakras: Zaela's Drake
- Iron Juggernaut
- Kor'kron Dark Shaman: Haromm and Kardris trained thousands of shaman to whisper reverently to the elements to requisition their aid. The army of Garrosh, however, does not ask -- they take what they desire in the name of the True Horde. Dark Shamanism forces the elements into servitude, twisting them into burned-out ash, corrupted waters, and toxic air.
- General Nazgrim: Once a grunt in service of the former warchief, Thrall, General Nazgrim rose quickly through the ranks after overwhelming victories in Grizzly Hills and the sunken city of Vashj'ir. Fiercly loyal to the Horde and bound by a rigorous code of honor and duty, Nazgrim will hold the line for his warchief until his dying breath.
- Spoils of Pandaria
- Thok the Bloodthirsty
- Siegecrafter Blackfuse
- Paragons of the Klaxxi
- Garrosh Hellscream: Warchief of the True Horde
Some of these are somewhat self-explanatory or have small expositions that give us some details, while four -- Nazgrim, Malkorok, the Klaxxi Paragons and Garrosh -- have journal entries that explain their encounters and thus give even more details. Nazgrim's encounter focuses around warrior abilities and his command of Kor'kron troops, while Malkorok seems to focus more on his position as Hellscream's champion, and the Klaxxi Paragons are clearly attracted to the power of Y'Shaarj that Garrosh now controls. Each of these fights that we can look at seem pretty detailed and complex, but we'll find out for sure once we have more information. (The Klaxxi in particular is a council fight that also draws from Megaera's mechanics).
The big news is the four phase Garrosh fight. It definitely seems as though Garrosh is the one in control, and not Y'Shaarj controlling him: while it's clear that Garrosh is empowering his soldiers with the essence of the Old God, the fight (based on the journal entry) seems to be more Garrosh using Y'Shaarj as a weapon and power source and less about being driven mad or corrupted by his power. Whether or not it ends up that way we'll have to wait and see.