In my campaign I have a magical disease/curse that infects people and especially affects magic users. Zombies were infected with the disease/curse and surrounded the players. The cleric casts Turn/Destroy undead, which succeeds on 5, but, using the death burst effect, explode and also force the cleric and ranger to make a dc 15 con save, which they fail and become infected with the disease. While this disease is crucial to the plot, and I overlooked the idea of the cleric exploding 5 zombies all at once, the cleric is upset. he believes that destroy undead should simply destroy the creatures and not let the effect be of use, the wording of the effect is, Death Burst. If the plague zombie is dropped to 0 hit points, it explodes in a burst of decaying flesh. Any creature within 15 feet of the plague zombie must make a saving throw against its Spellplague. But on the adverse, Starting at 5th Level, when an Undead of CR 1/2 or lower fails its saving throw against Your Turn Undead feature, the creature is instantly destroyed. After deliberation, RAW would make me believe that they would cancel out death burst, but also, as we are mid fight, I worked in the ability for the cleric to get healed, as he was obviously in a bad place, after his spell had a backfire to it. Should I change my campaign to appease the cleric, and restart the fight, or, should I continue the fight at the next session tomorrow, and tell him it is what it is? Is he in the right, or I’m at right to say that the effects can both happen at the same time? My narration, as it applies in the story, radiant and necrotic energy burst from the zombies. Basically his deity and a rival deity are competing. Sorry for the long post, just need opinions on what I should do tomorrow.

I think it's happened now, let it play - but maybe try and work something in that the Cleric can find to remove the disease/curse?

Destroy undead does not reduce the target to zero hp. It instantly destroys the target. It does not even technically "kill" the target. How it manifests depends on dm interpretation, but RAW, if the burst is triggered by either dying or being reduced to zero hp, destroy undead will not trigger it.

The whole point is he already knows how it works and it’s figured greater restoration does fix it, but hasn’t built an antidote, so he can’t heal himself. The paladin did, but he is still fussy.

For me... I would just retcon and say the burst didn't happen. I know it can throw off the story telling, but he used his ability intelligently in a way the countered the specific challenge he was facing and got punished for it. That sucks for him and he shouldn't have to face consequences for a problem that only exists because his DM is ignoring the agreed upon rules of the game because of a momentary miscommunication of how the game works.

That’s fair and I’ve considered it since he doesn’t seem to be happy, and I am new so my interpretation can tend to be wrong. But for specifics, this deity working against his is SUPPOSED to be eventually negating his abilities. I literally built the story for his one character and if it challenges him continuously and it upsets him, I feel I can’t keep bending the outcome for him. Basically no matter what my end goal is for him to eventually die so I can Rez him as a divine spirit, and without telling him that I don’t know how to make it okay to challenge him. His deity is supposed to be losing effect over the prime material throughout the campaign, and this is his first taste of losing abilities, and if this is how he’ll act throughout I feel like I’ve wasted my time working on a story for him. I feel like he has a problem from knowing him personally of feeling like he is losing. But again, I can’t tell him, but my whole point is to try his character until he eventually dies and so I can resurrect him back as the first epic hero/divinity rank hero of the party. My campaign ends in a huge war between gods and demons and this is supposed to be a challenging campaign. I understand now the ability of the spell, but given the circumstances I figured it was reasonable to not have full affect this time, as in next time maybe he does overpower the Necrotic energy with his own Radiant energy? If that makes sense? Idk this would be the second time I’ve tossed the fight (the first being because this same player said he didn’t know who a mindflayer was so the fight against a mindflayer and his minions were unfair) and I don’t think it’s fair to keep resetting fights because of one player, who again the campaign is centered around.

Okay, well... I think we might have some challenges because I'm fundamentally opposed to some of the things you're doing. Not that what you're doing is "wrong" or "bad", but it's just not how I like to play the game and I feel like it has the potential to go very, very badly very easily.

For one... it is  very challenging to have a "Plan" for your players. Especially this concept that you fully intend for the player to die. As a player... even if dying led to me being almost immediately resurrected in a more powerful form... I still wouldn't enjoy it. If I die I want it to be because I made bad choices, and not because the DM fully intended to kill me to satisfy the story that the DM wanted to tell. I would feel like my character and story has been taken away from me so that the DM could tell the story that they want to tell... at that point, why am I even playing? If the DM has a story they really want to tell and my input doesn't matter, they might as well just write a book and let me be who I want to be.

That's pretty extreme of a response, and I don't want to accuse you of doing anything wrong... but I just wanted to let you know that would be what would be running through my head if I was the player in this situation, even if I was wildly misinterpreting your intent. I do think that playing up the idea of his Deity losing power could be interesting, but again... on some level it would feel like being punished for my class choice. Is the Fighter losing strength for story reasons that are 100% out of his control? Is the Bard unable to sing despite doing nothing in-story that would result in them losing that ability?

I also wonder about this mindflayer fight. Why does he feel like that fight was unfair? Just not knowing that a Mindflayer would be there? That seems like the player is over-reacting, without knowing more details. I can understand, then, if you feel hesitant about ret-conning a fight if he's already used some kind of minor complaint to undo something that worked against him... it sounds like a challenging player for you to deal with. Again, I don't want to assume too much, in case there is some detail that explains things better, but based on what you said, it does seem like this is a problem beyond just this one Feature use that led to confusion between you and your player.

I don't know all the details so I can't tell you exactly what to do, but I will say this... be very, very careful about building a campaign around a specific character. If the player doesn't want to do what you assume they're going to do it will ruin your whole campaign, and trust me... your players will surprise you at every turn. I can't tell you how many times I've prepared for a very specific outcome and had to scramble when my players went completely off the rails. And also be aware of your other players at the table... how would you feel if you built a complex, interesting character, but your DM had built their entire campaign around someone else at the table? Wouldn't it feel kind of... insulting? To basically be an NPC in someone else's story? I might be interpreting this more over-dramatically than you're intending, and you might have equally complex plans for everyone at your table... but it's still a potential pitfall that you need to be aware of and you should be ready to adjust your story depending on the choices your players make, instead of adjusting the consequences of your players choices to fit the story that you want to tell.

Just let it happen, make sure there is a way for the pcs to rid themselves of the spell plague, but make it harder for magic pcs. I made a curse like this and did it like tht. It makes the game run faster and makes the characters make smart decisions 

I'm unsure of the mechanics youth asking about, so I'll refrain from commenting on those.

However, I'd advise you not to plan the character's agency for them. Don't say "When x happens, I'll...". That is likely to produce a lot of frustration. It's possible it'll all be fine, but it's likely it won't. If the dice doesn't go how you want it to or they don't make the decisions you expect them to, either you'll force a railroad and they'll resent you for trying to hijack their character, or you'll get frustrated and resent them for "ruining" your plan. We get posts on here where that's happened quite frequently. DMs try to force their narrative, the player doesn't cooperate, and then one or the other gets upset because the other is ruining their game. Don't try to force a single narrative on them, just have plans for what happens for each likely outcome.

That's not to say that you can't have plans. You can have a plan that if he dies, he will come back as a spirit. That's great. Just don't set your campaign around it. If his decisions and dice decide that he lives, then let him live and come up with a different plot hook. Rather than saying "When he dies..." say "If he dies...". Let the players direct the narrative. 

Quote from MrGsdaboss >>

... no matter what my end goal is for him to...

No. This is a bad idea. The DM controls the circumstances, the players control their characters, and those two things together distill into the actual story that develops. You really, really shouldn't mess with that. You set the stage, but you don't get to write the play. Let the outcome be what it is. 

People are so used to consuming scripted stories that they can think that there should be a way through.  Certainly, it's good if there can be chances but then you take your chances.  The character isn't dead and can seek a way forward.  

Another challenge is that character creation has become an involved thing leaving people to be invested in their characters.  In early editions of D&D you just rolled on a set wilderness encounter table and often, if your low-level characters didn't or couldn't run, you were likely in trouble.  The game wasn't metagamed around players.  It's a dice game in and like other games of chance, it's down to the player to assess the odds.

If you want to play by RAW then the undead should not have exploded.  If you want to play it so that undead will explode as a consequence of destroy undead then it's your game and that's also fine.  What I don't think would be fine is if the character didn't have any reasonable chance of warning of the potential danger. 

Are the players gung ho?  With the mind flayer, did they fail to ask relevant questions or make relevant checks to assess the situation?  As TransmorpherDDS has said, the normal thing would be that destroy undead would not trigger a death burst type effect,  Had you given any indication that this might not be the case in your campaign?  If not then the situation might be tough on players especially if they have an idea about how the rules normally work.

If you want to retcon, that could certainly work.  If you want to get the characters to press on then that's also great.  The situation may get them to make any relevant arcana, religion, history etc. checks ahead of time.  The characters are heading into the unknown and there can be consequences, especially if the characters don't proceed with potentially necessary caution.

They didn't previously know that in your world that undead might death burst as a consequence of destroy undead.  Now they've seen it happen.  As a result, unless you retcon or something, the cleric may be weakened for a while. 

You can (optionally) still say you could have (if relevant) made more checks, I could have given more warnings but what's done is done. 

The party can still work together to find a resolution.