It would be great if this was one of the "Condition" options on the D&DB character sheet.

Quote from GrumpyBATAgain >>

It would be great if this was one of the "Condition" options on the D&DB character sheet.

 Seconded. Similarly haste and bladesong 

Most spells require you to see your targets, but Slow is one of the few spells that doesn't have this requirement.

And unlike Fireball, it also allows anyone casting the spell to sculpt the spell to avoid hitting their allies, even when they can't see their allies.

The first part can be re-worded as, "Each target is affected for the duration of the spell unless it succeeds on a Wisdom saving throw."

The second part says when a Wisdom saving throw may be made. 

There is no conflict.

This wouldn’t work on the vehicles in Avernus would it?

The spell only affects "up to six creatures of your choice" (emphasis mine) so vehicles wouldn't be affected because they aren't considered creatures.

The creatures inside the vehicle can be affected though, which would definitely effect the operation of said vehicle.

While under the effects of Slow: If a creature casts a spell that has a duration of 1 action and rolls a d20, getting an 11 or higher, then 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. However, what I want to know is if at the end of the creature's turn its makes a successful save, does that mean that the spell it was casting then acts normally, or does that mean that the spell still doesn't take effect until the creature's next turn?

I'm definitely going to pick up this spell up for my halfling wizard at next level, because it will be insanely useful for fighting other casters. Taking away a spell casters reaction means I can continuously zap an opponent with lightning bolts or blow them to smithereens with fireballs. And it becomes even deadlier if I use a fifth level Bestow Curse to target wisdom or dexterity saving throws. 

And if they try to cast dispell magic to get rid of it, there's always counter spell. 

this spell is so underrated that it doesnt even make underrated spell lists

It is, my fourth level Bard was able to have it.

Perfect spell to pair with hypnotic pattern based on initiative. If you are going before the enemy, hypnotic pattern. If you go after the enemy, slow (due to no friendly fire).

Would the slow spell effect ageing? If it effects action economy (including racial traits like changelings shapeshifting) then the biological side is likely effected

So what happens if a creature that is slowed and tries to cast a spell but rolls below an 11. the wording is kinda confusing?

Quote from WolfRyder70 >>

So what happens if a creature that is slowed and tries to cast a spell but rolls below an 11. the wording is kinda confusing?

 

My interpretation of it was that if it rolls below an 11 the spell is wasted and nothing happens

If the roll is below 11, nothing special happens and their spell resolves normally.

Quote from Skillager >>

Quote from WolfRyder70 >>

So what happens if a creature that is slowed and tries to cast a spell but rolls below an 11. the wording is kinda confusing?

 

My interpretation of it was that if it rolls below an 11 the spell is wasted and nothing happens

Quote from Novus >>

If the roll is below 11, nothing special happens and their spell resolves normally.

 Ok guys thank you for the replies. I found this online it suggests that if rolled below an 11 the spell fails. https://explorednd.com/gameplay/slow-5e-guide/

Something that seems odd with this thing that magically slows down targets so they don't have time to react to things, their attacks are so slow they can only make 1, and they only get one type of action in a turn...
Yet their speed is not affected. So a slowed target might still outrun you if it's a race to a point.

It states that speed is halved.

A creature can technically still take legendary actions, so long as the legendary action isnt another attack. A adult red dragon can totally still use its Detect legendary action. No hiding for you!