The lore keeper can be found speaking to High Explorer Dellorah about Ulduar's current security. Of particular note is the fact that he refers to Yogg-Saron as being the most tightly guarded prisoner of Ulduar.
To start the Flame Leviathan event on hard mode, a raid member must talk to Lore Keeper of Norgannon and tell him to activate Ulduar's defenses. Upon doing so, the Lore Keeper disappears and High Explorer Dellorah runs off to warn Brann Bronzebeard of what the player has just done.
As this is the first npc most players notice it has been the cause of many wipes for new groups, as someone might start the event using this npc and none of the towers getting destroyed, thus making the players do the flame leviathan under the maximum hard mode setting.