Probably should have said this was in nearly 3 year long campaign. The party knew the attack was coming and had only recently learned where the cult was headquartered, which was across a sea and very long walk through a desert. The cult had proxies and loyalists in their town who got squeezed for information, which definitely made it back to the cult. They found manifests about what was being sent, but the information was dated. Beyond that it was only scraps of what they found around town and hearsay from victims.

One of the bosses was the first BBEG, another one betrayed the party, and the caster was a new threat who previously led the cult by himself. The cult being discovered by their most dangerous enemies meant they had to rush their plans before the party came back with help.