Scourgelord Tyrannus

This boss encounter will force you to battle Tyrannus on the ground, whilst his frostwyrm Rimefang gives him assistance from above. They both have unique abilities, making this an encounter where everyone needs to know what they are doing to not die a horrible death. 
 

Abilities

 

Scourgelord Tyrannus

 

• Forceful Smash - Inflicts  28275 to 31725 physical damage (47125-52875 on Heroic) and knocks the target back. 10 second cooldown.
• Overlord's Brand - This will brand a party member, converting all damage the party member does to whoever Tyrannus is targeting (usually the tank) and all heals done by the branded person will heal Tyrannus aswell. This lasts for 8 seconds.
• Unholy Power - Increases the damage done by Tyrannus by 75% but reduces his movement speed by 50% for 10 seconds (100% damage done, no speed reduction on Heroic).    

 

Rimefang

 

• Mark of Rimefang -  Marks a party member. This party member will be attacked by Rimefang's ability...
• ... Hoarfrost - Deals   4813 to 6187 (7875 - 10125 on heroic) to everyone within 8 yards of where the Hoarfrost hits. Will also freeze all that are hit for 5 seconds.
• Icy Blast (1) - A spell that Rimefang casts whenever he is not casting Hoarfrost, deals 4813-6187 (6563 - 8437) damage to everyone within 7 yards of where the blast hits, and also leaves a spot of ice on the ground.
• Icy Blast (2) - This is the spot of ice left on the ground by Icy Blast. It reduces movement speed by 75% and deals 2313 - 2687 damage per two seconds (Ticks every second on heroic).

 
If the tank feels he is taking too much damage by Unholy Power, then he can kite Scourgelord Tyrannus over to an ice-patch, which will slow Tyrannus aswell, and allow you to get away from his damage for a while. However, if your tank is well-geared , he can just take on the boss when the empower happens, and perhaps use a cool-down or two. 
 
Other then that, it's a pretty straight-foward fight, and when he's dead you will gain entrance to the last part of the Frozen Halls: The Halls of Reflection. 
 

Trivia

 

1. Tyrannus sometimes says "Power... Overwhelming", the famous cheat code from Starcraft.
2. He is also known saying "Perhaps you should have stayed in the mountains!", which is a reference to Thorim's intro, where he says to have met the player "In the mountains".
3. Tyrannus is Greek for "tyrant".