Thornir Alekeg
Albatross!
fusangite said:But what if they can inflict enough damage to kill the banshee in one round? According to the rules, readied actions always interrupt the action that sets them off.
Yes, that is exactly correct. Where is the problem with this? The DM should be aware of the readied actions his players declared and when they will be triggered. The DM should basically be each action in his head as they are declared. If the players readied appropriate actions, he should know that they will get to act before the banshee's attack.
In terms of knowing about the scream being an attack versus opening her mouth to parley, I would allow the party a Sense Motive check or Knowledge: undead (I don't understand the Knowledge: Religion references, are banshee's part of a religious organization?). If they succeed the description is adjusted: "You see the apparition open its mouth and in our gut realize it is about to do something nasty. Atrurus and Skith, take your readied actions."
If the DM forgets about the readied actions and rolls the effects of the scream, then when the players protest, you have to suck it up and back up time just a moment from "The creature lets out a piercing wail, roll a save." to "The creature starts to scream, but Arturus and Skith's sudden attacks disipate the creature and you hear the faint echo of a scream that make the hair on your arms stand up on end."