The attack on the Elleseandi embassy was a crucial event to the leadup of the Siege of Val Tanza. The official cause was never revealed to the public.
Apuccio, and later Cassandra's diary, revealed that it involved an arcane device emitting a magic feedback ripple through the building, which killed the ambassador Lady Aladriel, her Keeper Pronas, and all of her staff.
This device was planted here by Benicio in order to provoke an Elleseandi attack on the province of Val Tanza.
Cassandra managed to survive, and seriously wounded Benicio, who also got away.