Who called out Hastus, the cleric or the followers?
If it was the followers wouldn't they be the ones facing the 10HD minion?
I don't imagine most deities (or their minions) would go after their own worshippers. Yeah, the followers may have called him, but it's the Cleric who was the enemy.
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Back in 2E, I had "The Book of Tables", an old 3rd party supplement that had a random % table of actions the diety would take if called. I made several checks to see if intervention happened, but it never did - save once.
Once, in desperation, the party cleric called on his own god while the group was facing a cleric of Orcus and a few undead minions, and the cleric's deity answered. However, this was a climatic battle the PCs could handle, and I was miffed it'd finally gone off (after about 2 attempts/session when the characters found out they might get a response). The deity appeared, stopped time, chastised the cleric for calling on him. The cleric tried to explain "We're battling the priests of your enemy, Orcus" - to which the god winced at the mention of the other power, but he gave the cleric two "favors" (read wishes) to rectify the current situation.
Just as the cleric was mulling over how to word the two favors, Orcus likewise appeared, having been called by the cleric's mention, and was clearly miffed over the interference. Ignoring the cleric, Orcus got into a heated debate with the deity and finally announced that for every favor the cleric's god granted, he'd likewise grant a favor to each of the three priests (a greater priest and two lesser) that were present and loyal to him. Shrugging, the cleric's god replied that was why he disliked to directly intervene - opposing gods could do likewise for the other side, and it just made things complicated. He then asked if the Cleric had decided on his two favors.
Realizing what would happen, the cleric replied he'd thought it over and he really didn't need the help, and that he and the party could handle things as they were. In a flash, the two dieties disappeared, and the fight resumed, though the party cleric had found himself in a better physical position (his god had surreptitiously let the cleric remain where he'd moved while time had stopped - at the feet of the group's fighter who'd gotten low on hp in the fight).
I didn't have any problems with god calls from there on out, and the characters were
very careful not to use the name of opposing gods after that.