The Guardian

The Guardian was a revenant who warned Arthas to not draw the cursed sword Frostmourne from its pedestal, but Arthas did so regardless of the dire warning.

The Guardian is described by Arthas as "the leader [of the group of revenants], the one assigned to guard Frostmourne" or as "the strange protector", and the other revenants "the other elemental guardians of the sword". Little information is given beyond that and the description of the battle. The only other hints to his motivations are the inscription on the dais being written in Kalimag, and the green ore in the walls of Frostmourne Cavern being referenced as "softly glowing turquoise crystals and veins of ore".

Deathbringer Revenants could be found in Frostmourne Cavern, as well as Frost Revenants and Skeletal Marksmen.

Quotes

 * Intro
 * The Guardian: Turn back, mortals. Death and darkness are all that await you in this forsaken vault.
 * Arthas: I doubt there is anything down here more terrifying than what we've faced already.
 * The Guardian: ''Believe what you will, boy. You shall not pass.


 * Defeated
 * The Guardian: Turn away... before it's... too late.
 * Arthas: Still trying to protect the sword, are you?
 * The Guardian: No... trying to protect you... from it.

Speculation

 * The green ore in the Cavern could be Saronite, suggesting that The Guardian may have been connected to Yogg-Saron.
 * The revenants, having a sort of resistance to the Lich King's influence, would explain why he tried to warn Arthas instead of encourage him. It's possible that he saw the future Arthas would bring as the Lich King and tried to prevent it. Alternatively he could have feared Frostmourne being out in the open would increase the Lich King's powers to the point of allowing the Lich King to bring the revenants under control.