Holy King's Tomb (HKT) is the seventh dungeon in the game. It is much less tedious than the last dungeon, Roaring Maw. The dungeon is located in Land of Sighs (X:801, Y:56). This Main Quest requires mobs and item drops.
You acquire the first quest at level 56 called [L56. Posion Gas in the Holy King's Tomb] This quest requires you to talk to White Flame Knight Lilia in Land of Sighs (X:445, Y:559). Once you turn in that quest she'll give you another quest [L56. Infiltrate the Holy King's Tomb] which requires you to kill Urkate and defeat 20 Crypt Scorpions.
Once complete, turn the quest in to Holy King Narmi. It will then give you the quest [L58. The Holy King's Ring] which requires you to defeat Tolos and collect Ring of the Holy Key from him, and also kill 18 Ancient Tomb Thiefs and 6 Ancient Tomb Theif Mages.
Once complete, turn the quest in to Holy King Narmi and it will give you the last quest for this Main Quest [L60. The Holy King's Guardian Beast] which requires you to kill Jafashazar, the last boss in the dungeon. His portal is at (X:78, Y:306).
Once you complete the last quest, turn it back in to White Flame Knight Lilia in Land of Sighs and you're done.
There is another boss in HKT, Brachast, but you only need to kill him if you want the credit and Mission Order title. The title is [Steward of the King's Tomb] (Cast Speed +10%, Max HP -10%)