Alright some thoughts about the Frost hidden appearance:
1. Something to do with Koltira. There was a quest series back in LK about translating a book written in the "Language of Death," which only Koltira could read. So I'm going to put him as my bodyguard for the near future, and maybe investigate the LK vision places. Btw the "Language of Death" is apparently written in "strange symbols," see here.
2. Rare spawns: there is a mob called the "Nameless King," and one called "Devouring Darkness," which both fit the Lich Kings motivational speeches.
3. The most obvious, and least exciting, would be that this is a drop from Tichondrius "the Darkener." Let's hope not.