The Royal Apothecary lead by some new character is a side project of Varimathras'. Their intentions have always been evil while Sylvanis' direction of the Forsaken has always been out of vengeance against their torturer - she is basically good.
The Royal Apoth. release some plague at the Wrath Gate and it kills Bolvar Fordragon(sp?) this brings the king of the Alliance (the rightful one, not the kid) to declare war against the horde (officially). Apparently before/during/after this some I believe either Arthas' army of that new race, whatever they're called they are in the vid, either start fighting the Horde or they try to stand up to Arthas. Either way of what I've heard Arthas kills Saurfang the Younger. These events lead to Alliance infiltrating the Undercity which is now taken over by Varimathras and the Apoth. and all the forsaken take refuge in Orgimmar. Alliance go and kill Putress or whatever, the new character behind the Apoth. with Varimathras while the horde go somewhere to kill Varimathras, I forget where.
No events happen from this except:
1. Bolvar is dead
2. Saurfan the Younger is dead
3. Alliance officially wage war on the Horde
4. Varimathras is "dead"
5. Putress (who cares..?) is also dead and the Royal Apoth. may be disbanded or their intentions changed - IDK which.
It's pretty cool especially since each side get to fight with 2 of their leaders. Horde do the quest with Thrall and Sylvanis and Alliance with the new king and Jaina.
Then again with the Lore the way it is most if not all of these characters may or may not remain dead. We already know Dread Lords are like, immortal and since Magni Bronzebeard is some how alive again I can imagine the others being revived too. It's best not to ask questions.