During Elder Tilrith's brief visit, the two of them sat down and had a little chat.
Rex learned that she came to him wanting to make a deal in exchange for him eradicating the Rastrikan Demons that were filled with outlaws, too stubborn to listen. She passed the task to him by saying that it wouldn't look good to her image as the new ruler.
Eventually, the two establish a deal as Rex also has a favor to ask from her.
Albeit doing it discreetly, Elder Tilrith would try to convince the high-rank Supernaturals races to help with one part of Rex's plan against the Executor. Realistically speaking, if some were not on board, she would make sure that they at least would steer clear from the way.
Calling it a favor, however, is not fitting.
Since what Rex wanted would also benefit the Supernaturals, it's more like a contribution.
But thinking back, he realized that the deal was not what it seemed.