FormerlyHemlock
Hero
I have been intrigued by this idea since you posted about it, and I periodically think about using it, but when I think through scenarios it seems like much more often than not, there are enough dependencies that everyone would end up rolling initiative anyway. So if you don't mind, since you brought it up in this thread, take the situation as described in the OP and for simplicity assume that no one is surprised. The NPC is casting an AoE that will potentially affect all the PCs and if he has a lick of sense he will try to move to cover after casting. I find it hard to imagine any reasonable actions on the PCs part that would not have me wanting pretty much a total ordering for resolving actions. If you are willing to indulge me, would you sketch out how this scenario could play out without the need for the NPC and all the PCs to make initiative rolls? Choose whatever actions for the PCs you would like or assume a couple try to close to melee and a couple make ranged attacks and then move to cover. This seems like a bit of an ask, but if your willing to spend a few minutes on it, I would be appreciative.
This particular example is extremely simple, since there's only one NPC and he's the first one acting: he casts his spell while all the PCs are on implicit Delay (negotiating), and then all the PCs take their turns. Since there are no other NPCs in the combat, PCs can basically just take their turns whenever the players decide what they want, and so I'd probably just let them do that: once you roll dice, that means your action has taken effect and you can't take it back. (In rules terms this means that I'm also assuming an implicit Delay for any player who is slow to declare his action.) Once everyone has taken an action, the round ends.
I suspect what you're really interested in is the SECOND round, when the NPC does not necessarily go first. (If this is what you meant by "no one is surprised", then for you this would be the first round.) I suspect you're right and that, since the enemy is moving away, it might require initiative rolls all around. Let me work this out real quick:
DM: [to the players of Rath and Rupert, Int 9 Dwarf Fighter and Int 11 Half-elf Ranger respectively] Okay, guys, you're not sure what Shoalar Quanderil is going to do this round but you need to make up your minds. Rath, what are you doing?
Rath's player: Chaaaarge! I hit him as hard as I can, of course. [It's been established by long practice that what Rath says 'Charge', he means 'move and attack if possible, otherwise Dash to get as close as possible.'] And then I action surge to hit him some more, if I can. [It's also been established that Rath's player is using GWM whenever he says "hit him as hard I can can".]
Rupert player: Can I shoot over Rath?*
DM: Sure, he's short.
[* This exchange has become something of a ritual for this play group.]
Rupert: Okay, then I'm shooting Sholar Quatrone and ensnaring him with Ensnaring Strike.
DM: Well, Shoalar Quanderil casts another Tidal Wave right on you, Rath, and Delsenora. [points to map] Then he jumps off the pier into the ocean.
Delsenora: Except that can see what he's doing, and I'm Blinding him and then withdrawing 20', back onto the bridge, so I won't get caught in the same wave as Rupert.
DM: Okay, then it might only get Rupert if you're fast enough. Let's resolve.
Rath: [so ready for this--he's already rolled while everyone else was talking] Wham, wham, wham! I hit him three out of four times for 63 points of damage! Do I kill him?
DM: Not quite, but he's definitely hurting if those blows landed. What's your initiative? [roll, roll] It seems that you did barely manage to hit him as he was still casting Tidal Wave, so not only do you escape his spell, but instead of just diving into the water, you basically backhand him into the water with your axe and jump in after him!
Rath: Yesssss! [notice he doesn't object to following the bad guy into the water--charging is his Modus Operandi.]
Rupert: I missed him, twice.
Rath: Awwww. So he's still alive.
Delsenora: [rolls dice] I lost initiative so Quandros Whatsisface, so, yeah, I get hit with Tidal Wave.
DM: [rolls] 19 points of damage to both you and Rupert, DC 14 Dex save for half.
Rupert: [rolls] I dodge it! Only 9 points of damage for me.
Delsenora: [rolls] I dodge... poorly. 19 points of damage for me, and no Blindness because he's underwater, right?
DM: Right. You don't manage to get your spell off--deduct no spell points, but he doesn't have to save.
Delsenora: [ruefully] I should have Delayed instead, or Counterspelled.
DM: So, to recap, it's been twelve seconds since things exploded into action. Delsenora is coughing up seawater, Rupert is cursing his aim and blessing his nimble legs, and Rath is kicking his legs furiously to remain afloat, and menacing Shoalar with his great big axe. Shoalar is starting to look alarmed at this turn of events. Declare actions for round three!
[and off they go]