But not a simple one. She's a void Dreadlord. We've seen the light version on the Vindicaar so it makes total sense that the old gods would use them as well.
The real Thallyssa died by the dagger in Suramar city and the void Dreadlord took her place to play out the forces of Azeroth and the Legion against each other.
How would Thallyssa know that there are potential allies out there to help her cause? She wouldn't as someone who was a a bubble for 10 thousand years but a Dreadlord would know it.
Deceiving others to do her job while standing in a save place for the whole time? That's a thing a Dreadlord would do.
Attacking the night elves after they helped liberating her people? That's a thing a void Dreadlord would do to take revenge for his masters for the An'Quarai war.
Let's face it. The horde now got the Dreadlord they so desperately wanted to see in Jaina.