Saphhiron was once a part of the Blue Dragonflight until he was slain by Arthas on the lands Northrend (depicted in Warcraft III: The Frozen Throne). Kel'thuzad, a former Lich, then ressurected Sapphiron, now he resides in Naxxramas, Kel' Thuzad's lair, to protect him. He is the second to last boss in Naxxramas. Naxxramas was originally a 40-main raid, but then was reintroduced in World of Warcraft: Wrath of the Lich King to be a 10 and 25 man raid dungeon. All bosses, including Saphhiron, were tweaked in the way the encounters occured to adjust to the new level cap and other things.
In WoW: WotlK, Saphhiron drops the items to unlock normal and heroic Eye of Eterniry, another 10 and 25 man raid dungeon.