You alter time around up to six creatures of your choice in a 40-foot cube within range. Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration.
An affected target's speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can't use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn.
If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.
A creature affected by this spell makes another Wisdom saving throw at the end of each of its turns. On a successful save, the effect ends for it.
* - (a drop of molasses)
I once cast Slow on 4 Glabrezu enemies (3 slowed 3 rounds and 1 for 2 rounds). Glabrezu = 4 attacks, +9 to hit, average 15 DMG on first two (plus grapple) and 6 DMG on the second two attacks + they can cast spells.
3 of them attacked (once instead of 4) while slowed for 3 rounds straight = potential 27 attacks saved. That alone saved our party about 243 Damage if they would hit each time... (and with +9 mostly they did...).
The one who was slowed 2 rounds cast a spell (darkness) in those 2 rounds instead of 1 round. Which saved us 1 round without Darkness, and the second round of casting would actually be another spell or 4 attacks...
Plus we hit them several times closely matching their AC (which would have missed without Slow) plus since they had no reactions, which saved us I think 2 times from AoO.
So potentially nearly 300 damage saved with one 3rd level spell! A must have.
My 6th level War Mage cast this on a Young White Dragon. The net result was a dead dragon.
Haste: MEGABUFF you or your ally.
Slow: MEGANURF your enemy.
Fantastic play, bet the party was chuffed!