In life, Nathanos Marris was the first and last of the human ranger lords, trained by the high elves in the ways of war. He was a disciple of Sylvanas Windrunner, formerly Ranger-General of Quel'Thalas and now the Banshee Queen of the Forsaken.
Nathanos's accomplishments were unprecedented. He was a tactical genius, responsible for Alliance victories spanning a decade of conflict.
Sensing the encroaching turmoil that overshadowed Lordaeron, he sent his younger brother Thereas to the kingdom of Stormwind, making sure no harm would befall him.
When the Scourge came, he defended himself at his house, but the mighty abominationRamstein the Gorger slew him. Nathanos became a mindless servant of the Lich King until he was freed by the Banshee Queen. He took the name Blightcaller and was given the title Champion of the Forsaken.
His presence is discovered by the Alliance after five of the best SI:7 agents are sent to investigate the Marris farmstead. Four never returned, and the fifth made it back to Stormwind only to end up dead in his sleep three days later.
Once it is determined that Nathanos is the Blightcaller, King Varian Wrynn enlists Alliance adventurers to gather an army and destroy him.
Cataclysm
This section concerns content exclusive to Cataclysm.
Nathanos will be absent from the Eastern Plaguelands in Cataclysm. A quest refers to him as the "former occupant" of the Marris Stead, and many of his quests are now given by other NPCs in the zone. He has been relocated to Undercity where he will let hunters unlearn their talents.
One strategy is to take Nathanos down as fast as possible, whilst reducing the aggro on weaker party members. This enables the group to at least complete the quest, or gain honor under the Honor system for the kill, while all the party is (still) alive.
You need at least 8 people in a raid group at lower levels, with a main and off-tank.
Focus on Nathanos himself and take him down as fast as possible. He can be taken down in a minute or so, at which point no more skeletons will be added. The Blighthounds will not do any serious damage until he is down. Deal with them second, and the skeletons third.
Avoid healing as 2-4 skeletons are attracted to the healer. This includes direct heals, heal over time spells, Warlock's drain spells, bandages, potions, pet heals, and life stealing enchants. Rather than heal, run; stop fighting to reduce threat (trinkets etc with Spirit bonuses are useful here) and heal normally; or let your tanks do the healing to attract aggro back.
It is easy to solo him at level 80, especially if you are a tank.