The Tsaesci are discovered in hiding in an Ayleid ruin. It is evident that the attack on the Imperial City was not a random thing that was, instead, planned in advance and is not likely a singular event unlikely to repeat.
Beginning at the ruin of Vilverin, for entirely unrelated reasons, the party begins exploring Ayleid ruins near to the Imperial City. Each step along the way, they discover more of the Tsaesci's influence in the form of corruptive mushrooms planted by the Tsaesci to create an army in forward positions in Tamriel.
From Vilverin, the party explores Fanacasecul, Piukanda, and Narfinsel and ultimately wind up in a small village known as Hackdirt. Within the ruins of Narfinsel and the village of Hackdirt, evidence of Draconic breeding is found on top of the mushroom efforts.
The party destroyed each of the mushrooms they came across, fought a few Tsaesci, dealt with a great deal of corrupted creatures inhabiting the ruins, killed a few dragon wyrmlings, captured a Tsaesci, and snatched up a few dragon eggs.