Gul'dan is the last enboss of Legion.
We never killed him in Nighthold. Look at him in p3. he is a demon. All legion commanders are demons, there are no exceptions!
Also his power was tenfold once at the tomb, thats not the weak Gul'dan who died there at the Tomb from our universe. He was turned into a demon beeing and empowered by either KJ or Sargeras.(he uses sargeras abilities in the NH fight)
Demons can slowly reform in the twisting nether or instantly resummoned.
Gul'dan might get even more power on Argus, so he rivals his masters KJ/Archimonde. Sargeras will empower him or he just tooks over the place, while his masters are MIA.
So that is poetry: It started with Gul'dan it will end with Gul'dan. We stopped him at NH for a while. Usually endbosses have big cinematics, too.
Not always true, but most of the time it is. WoD might be an exception only because devs did cut major content and changed the lore while the game was out.