Lord Marrowgar is the first line of defense against those that would seek to enter Icecrown Citadel and attempt to assault the spire upon which sits the frozen throne. He is the only known Bone Wraith in existence and is an undead construct similar to abominations. What is he constructed from? I'm glad you asked.
The Lich King and the scourge have killed a lot of people in the last few years. Most of them were probably raised as ghouls or skeletons and thrown right back at us among the hordes of undead already attacking, but some of them, quite a few of them actually, were reserved, and the bones of those adventurers were used to create what we know as Lord Marrowgar.
That's really it. He's a collection of bones pieced and held together by the power of the Lich King, and when you make him angry, he yells BONESTORM! and spins around a lot. This leads me to believe he's made up of mostly warrior bones, and maybe a few frost mages in there too.
As far as bosses go, Marrowgar is pretty light weight. Seems the Lich King needs to make his guardians a little stronger, but the king does have other responsibilities. That's why he's got scourge like Professor Putricide on the payroll. But more about him later.
Yes, you're eyes are not telling you wrong. It's a she-lich. The only one we've encountered so far, too. Lady Deathwhisper is the Supreme Overseer of the Cult of the Damned which is somewhat surprising since the cult has been around for some time, but this is the first we've seen of Lady Deathwhisper. Within Icecrown, Deathwhisper stands in the Oratory of the Damned behind the spire, preaching the will of the Lich King to the cult members that gather there.
The Cult of the Damned, for those that may not know or recall, is a large group of living (not dead or undead) races that have devoted themselves to the service of the Lich King. It was through this cult that the Lich King first exerted his influence in the world many years ago. Kel'thuzad was on of the founding members before he was killed and raised as a lich, and it was through the cult that the plague was brought to Lordaeron. It appears that Lady Deathwhisper's role has been to oversee the operation and to preach the message of the cult to the uninitiated. We don't know anything about her prior life, who she was before she was a lich, or how she came to be the overseer.
When you happen upon her, you can hear her extol the virtues of death and service to the Lich King. She's somewhat of a zealot herself, proclaiming even upon her death that it is all in accordance with the Master's plan.
There's no actual boss to talk about here, but that's not to say there isn't a lot of lore to talk about. For one, you have two very strong lore figures in the captains of the horde and alliance gunships, as well as the entire storyline leading up to the Horde vs. Alliance standoff in Icecrown to begin with.
Muradin Bronzebeard is the captain of the Alliance ship, the Skybreaker. Muradin is brother to Brann and Magni Bronzebeard. You remember Brann as the explorer from Halls of Stone and Ulduar (Evasive Action! EVASIVE ACTION!), and Magni is the King of the dwarves over in Ironforge. Muradin is the middle child and was the liaison between the dwarves of Khaz Modan and Lordaeron. It was through this connection that he met a young Arthas Menethil and helped to train him as he was growing up. Later on, Arthas would betray Muradin's friendship when he took up Frostmourne and seemingly killed Muradin.
Muradin survived however, and found his way to the Frosthold in the Storm Peaks where, his memory lost to him, he joined and became king of the Frostborn Dwarves. Through a series of quests in the Storm Peaks, we the players reunite Muradin with his brothers and he gets his memories back, and first on his list of things to do is to take down Arthas who had since become the Lich King. Thus we see Muradin take a central role in Icecrown as he seeks to end the reign of Arthas that started when they had found Frostmourne.
On the Horde ship, Orgrim's hammer, High Overlord Varok Saurfang leads his forces. Saurfang has been a fixture in the Horde since early WoW, serving a s a quest turn in for Nefarion's head and backing the rule of Thrall over the new horde. He is a veteran of the first, second, and third wars on Azeroth, and a legend in his own right. He lead the Horde forces in Ahn'Qiraj, and served as an advisor to Garrosh Hellscream in the push into Northrend.
Saurfang's son, Dranosh, also fought in Northrend at the Wrathgate. It was there that the Lich King took the soul of Saurfang Jr. And it is for this reason that the assault on the citadel is very personal for Saurfang Sr. In fact, the battle directly after the gunship battle is against Deathbringer Saurfang, the broken spirit of Dranosh, now in service to the Lich King. As the Alliance, we get to see a scene play out after the fight where Saurfang comes to reclaim the body of his son. It's a touching moment (until Jaina mucks it up), but I imagine it's even more dramatic Horde side, as Saurfang has to issue orders to attack his own son, even if his spirit is no longer whole.
So two heroes of their respective factions, both with personal vendetta's against the Lich King, as well as a mandate from their superiors to attack the citadel. The question stands out, why aren't they working together?
The tension between the Horde and Alliance has always been tenuous, but with the events of the Wrathgate, the hostilities throughout Northrend, and the unwillingness to compromise shown by their respective leaders, both sides feel that they are more entitled to the defeat of Arthas, and rather than join forces, they fight each other on their way to defeat the Lich King. It's a matter of pride, and an unwillingness to trust the other side, but be what it may, this is how things have transpired. It's interesting to me that Tirion and the Knights of the Ebon Blade have managed to band together in the Ashen Verdict, yet while they spear head the attack on Icecrown, the Horde and Alliance trivialize that union with their squabbles.
It will be interesting to see how this plays out as we reach the end of the Wrath of the Lich King and what it means for the hostilities between the factions.
Saurfang is the Lich King's most powerful Death Knight. He's also one of the more recent acquisitions of the Lich King, which leads me to believe he's not powerful because of his service to the Lich King, but because of the strength of the soul from which this death knight was made. So let's take a look at how Deathbringer Saurfang came to be.
Dranosh Saurfang is the son of Overlord Saurfang (who we talked about on the boat). His mother is unknown. Our first encounter with Saurfang the Younger is in Nagrand where he is one of the soul survivors of a war party made of young orcs on a mission of revenge against the Murkblood Broken. The broken had decimated Sunspring Post, and young Saurfang's band did not fare well when they confronted them. Through a few quests, you assist him in his revenge, and when you finish, Saurfang Jr. declares that he cannot return hom, and that he must walk the earth, for he feels he has a greater destiny.
Fast forward to Northrend. Saurfang the Elder and Grom Hellscream are leading the Warsong Offensive in the frozen wastes. Saurfang the younger is stationed at the Wrathgate. In a role similar to that of Bolvar Fordragon on the Alliance side, Saurfang Jr. leads the charge for the horde against the gate. The Lich King emerges and kills Saurfang the Younger, drawing his soul into the cursed blade Frostmourne. Before much else happens, Putrice busts out a plague on both the scourge and the alliance and horde forces, sending the Lich King packing back to Icecrown.
We don't see it, but it becomes apparent that the scourge were able to grab the body of Saurfang Jr. before they retreated as when we encounter him in Icecrown, he is alive as an undead orc Death Knight can be. But before Icecrown, there is one more glimpse of Saurfang. In the fight with Yogg-Saron, players encounter three visions. One of these visions incorporates the Lich King torturing the souls of those he has killed. One of them, the immolated champion, has not yet been broken, but the other, the turned champion, has been broken, and is in fact Saurfang Jr.
So a sad tale for one who once saw he had a great destiny. His soul captured and tortured, he was raised once more to fight both the Horde and Alliance as the breach the upper spire of Icecrown Citadel. Upon his death, the Alliance sees Saurfang the Elder approach for the body of his son. King Varian sees fit to allow this, over Muradin's objections. We're all so proud of him.
In the Horde version, Saurfang is there at the beginning of the fight, stating that he is there to claim the body of his son, who died at the wrathgate. He then reveals Saurfang the Younger's first name, Dranosh, which means Heart of Draenor.