All Cast (5e Variant Rule)
From D&D Wiki
All cast can only be initiated when an enemy is surrounded by PCs within range for all their spell attacks and is unable to move in any direction far enough to escape this range in a turn. A PC must use their entire turn (including action, bonus action, movement, etc.) to initiate all cast. All PCs that are casting designate the target and then roll their respective spell attack's damage die. The total of the rolls is then dealt with their respective damage types to the targeted creature as the PCs spells all converge to brutally barrage the target on the initiating PC's turn. As this is a very draining coordinated move, a PC may only initiate this tactic once before needing a long rest to use it again. This is a very powerful mechanic that is advised to use with caution.