When I researched who should/could be behind Mok’tringar, I found that most demons have a 3 expansion timeout policy from Blizzard when they are defeated. Archimonde is killed in BC, we have WoLK-Cata-MoP and he’s back!
Varimathras was involved up until Lich King and went away so he seemed due for a return. His comments about Sylvannas are related to his forced servitude in Undercity for a time. He begins to give a sense of what is going on before the pre-event, that plans have been made and are fast approaching.