Long story short, I was the DM running my adventure when one of the players asked if he could DM for a few session. In the last session, the level 6 party acquired a few magic items, one of them being:
Staff of Crowds
On a spell cast, you may impose that all enemies affected by that spell has disadvantage on ability checks made against that spell, once per day.
(By "ability checks made against that spell" he means saving throws)
This one worries me quite a lot. I'm thinking about only allowing the disadvantage against the damage from the spell, making an aditional normal roll if the spell has any secondary effects. As it is, this can be too much of a game changer if used with crowd control spells like hold person or high level charm spells.
Another thing is that this will probably be used by a druid, which means that if he uses the effect of the staff with say, a moon beam, he will probably stay the whole fight just moving the moon beam because that might be his best option to do lots of damage. I might be wrong here, but I don't want to encourge him to bore himself out in a fight.
The party is only level six now but soon they'll be seven, and with level 4 spells comes some pretty good AoE. This might not be a problem now but I think it'll be soon.
Is this item balanced for a lvl 6-7 party with the adjustments I made?