We never really got an explanation anywhere for Taldaram's activities in OK but my personal fanon/headcanon was that his territory was central Dragonblight as a liaison to the Nerubiasn, similar with Keleseth and HF/the vrykul.

But yeah, Lana'thel was high up in the Scourge hierarchy. I was really disappointed that Horde at least didn't get more quests that really explained the connection with the blood elves.