This is something we will have to work out in character. Hopefully Velon can use Lay on hands to help. Lia will have her healing ability cut in half if too many spell slots are used. I have a feeling that they will be needed.
Just suck it up and let's go. If you roll poorly, so maybe you automatically fail, or hit the wrong target. Yeah, one or two of us might take some extra damage. Honestly, if that happens let's hope that Poljen is the target. He can handle quite a bit, especially if it is normal damage. They know we're coming, so ignore the alarm ward. If we flood the lower part of the lair, so be it. Maybe Mari disables that water ward, maybe not. Let's find out… and let's waste this dragon and her pet elf-prince!
I just realized, the elf prince should be short and clad in purple… Prince!
I think some you are underselling how bad a 10% chance is of doing nothing on top of another 10% chance of helping your enemies is. That's a 20% or 1 in 5 chance of something really bad happening. Across 5 players, that means on average 1 person per round is losing an action or helping the other team.
Even if someone relatively benign such as Perry casts a DC 18 Hold Person on a party member or, Sune forbid, Polymorphs an enemy into a giant ape, what do you think is going to happen to the party? Nevermind Velon going nova on someone. In my opinion, unless you all want to walk into an almost certain TPK, Lia has to sacrifice 5 spell slots or we need to find a way to wait out an hour for this effect to pass. Also important to note, in character, the only one who currently knows anything about what is affecting us is Lawrance. So we would need to roleplay this problem and the solution.
Guidance and Bardic Inspiration aren't going to help too much here either with the sheer number of folks affected. Perry only has four Bardic Inspirations and Lia would be giving up her action (I.e. Rolling a 2 out of 10) to save someone else's, except she still has a chance to fail even that and give the bad guys a blessing instead. So you auto fail Lia by wasting her turn casting Guidance, and have a 20% chance of failing even that.
To be fair, Lawrance doesn't know what's happening, he just identified one of the ingredients. I was waiting for Dai to specify the effects and then was going to role play some of it, but everyone who failed to save sees these phantasms and distractions. I also think we should wait out the hour, which is what I will post RP, as well. This is very debilitating to the party's effectiveness.
Yup, bad news bears all around, because Math. It's not just about HP, as Seath said, we could drop major spells on the wrong person, killing an ally or buffing an enemy, never mind simply the wasted turns and spell slots even if the end result isn't so bad that turn. Hitting Poljen for a few HP might not be bad, but Banishing him, catching him in Hold Person, Smiting him for 60+ HP plus booming blade, or giving Neronvain an extra 100+HP buff for at least a turn by dropping an unintended poly on him certainly would be.
Moreover, Velon doesn't know any of this but is waiting for those whose job it is to disarm magical and mundane traps and wards to get him out of the water asap. I think he's starting to see things… uh oh… posting incoming…