About the Crimson Deathcharger
The Crimson Deathcharger is a ground mount that requires level 20 and the Apprentice Riding skill. With just the Apprentice Riding skill, this mount will increase your speed by 65 percent; if you have trained the Journeyman Riding skill as well, this mount will increase your speed by 100 percent. This mount's model is similar to the Acherus Deathcharger, a mount restricted to characters belonging to the Death Knight class. The Crimson Deathcharger has a dark grey body and red armor rather than a light gray body and purple and grey armor.
The Shadowmourne Quest Chain
If your character belongs to the warrior, paladin or death knight class, you can obtain the Crimson Deathcharger mount directly by completing the Shadowmourne quest chain. This quest chain begins with the quest "The Sacred and the Corrupt," given by Highlord Darion Mograine within the Icecrown Citadel raid instance; it requires collecting various materials, killing the bosses within Icecrown Citadel and completing specific tasks while doing so. Once you have completed the quest chain, you will obtain Shadowmourne, a legendary weapon. Defeat the Lich King, the final boss within Icecrown, while wielding Shadowmourne, and he will drop a Sealed Chest. The Sealed Chest contains several items belonging to different non-player characters; deliver these items to obtain a series of rewards, among which will be a Crimson Deathcharger. The Sealed Chest will drop only once; if you want to obtain more than one Crimson Deathcharger this way, you will need to complete the quest chain on a second character.
Purchasing the Crimson Deathcharger
If you can't or do not want to complete the Shadowmourne quest chain, you can still obtain a Crimson Deathcharger by purchasing it from another player, either directly or through the Auction House. Because of the extreme rarity of this mount, it may take a long time to track down a player willing to sell his; you should expect to pay several hundred thousand gold for this mount.
Considerations
Because this mount binds when used, any character of at least level 20 with the appropriate riding skill can learn it. However, due to the rarity of the mount, you should carefully consider which of your characters you want to give the mount to. Once you have used the mount on a character, only that character will be able to ride it; having such a rare mount on a character you never play would be unfortunate.