Before I open my question I like you to watch these two small clips:
My question is simple: Who or what was that thing within the "vault" that obviously Gul'Dan opened?
On WC3 there was a section where Mayev Shadowsong was hunting Illidan there and they were reading those writings on the walls where Gul'Dan said that "Sargeras had betrayed him" .. or such and now we have this scenario where Gul'Dan from another reality faces KD who says Gul'dan was punished for betraying the Legion.
So was this red fella at the first cinematic just some random demon or Kil'Jaeden or manifestation of Sarg himself or what?