Zul'drak Sentinel is a level 77 rare elite drakkari golem with approximately 45.5k health. He can be found patrolling around the Argent Stand and north of Drak'Tharon Keep in Zul'Drak. (I have indicated his patrol locations in green on the map below.) He is one of four rare spawns in this zone. The others are Griegen, Terror Spinner and Gondria.
This rare is part of the Northern Exposure and Frostbitten achievements. He drops Eternal Observer's Legplates which are level 74 bind on equip plate pants with a random enchant. Like other Northrend rares, he also drops Abandoned Adventurer's Satchel, which generally contains about 20 gold, a full stack of frostweave cloth, and often times a few crystallized elements. He's also worth a hefty amount of experience for people leveling in the area.
Zul'drak Sentinel was cake to kill as an 85, but I have run across him on leveling toons and he is definitely no pushover. He hits pretty hard and will use a special ability called "Whammy" which will hex you into a frog for 3 seconds and hits for a significant amount of nature damage.
He is immune to being rooted in place but he can be slowed with a mage's frostbolt, a death knight's chains of ice, etc... For levelers coming across him I recommend kiting him if you are ranged. Even if you can't get some sort of slow off on him he still walks at a pretty slow pace.