But the monster you are fighting can see right through it too.
That's fine.
Darkness is just a bonus. You can kill things without it.
And you have 4 party members in your example and you have to pass it with all of them (or half of them if you use the group check rules)
you just need a scout.
And you need to be obscured to even get a check, which you are not even inside the darkness.
At what range does the dragon spot you if your coming across a wide open field?
Except he is 55 feet away and it costs half your movement to stand up.
No. It never got a chance to move.
It is not a reaction. It can fly as a legendary action between the Monk's turn and your turn
No. "The dragon can take 3 legendary actions, choosing from the options below".
None of those options are moving.
https://www.dndbeyond.com/monsters/16782-ancient-red-dragon
Most boss enemies you are going to fight at this level are going to have legendary actions and being within 15 feet is going to hurt even if they can't move away.
Again, dead creatures deal no damage.
The boss I was fighting tonight had a DC23 dex save legendary action that was an AOE that does around 40 damage. That means sure - beat her initiative and get drug by another PC to within 15 feet and then you get to save and get to save again for concentration.
Mind Sharpener
When the wearer fails a Constitution saving throw to maintain concentration on a spell, the wearer can use its reaction to expend 1 of the item's charges to succeed instead.
But that's only for emergencies.
That is why I asked to provide an example of monsters where this will likely to work. There are not many.
If this doesn't work, neither will 1/3 the other classes.
I don't think WotC monsters negate melee build very often.
If the Monk drags you within 15 feet it will damage you.
ok. So the monk drag you within 60", and walk the rest of the way at the start of your turn.
Moreover breathing on the caster will likely end his concentration on CME, even with absorb elements.
Your still assuming it gets a turn.
The Monk used his action to drag you up to the Dragon I thought?
New monk feature lets you drag friends around as a bonus action + dash.
you game and they have a chance of killing the Dragon before its turn, but not before it "goes" because it "goes" with a Legendary action after the first person in initiative regardless of who that is.
Ok fine.
You're level 17 (get that last EB blast).
The scout lets you know there is a dragon up ahead. The party buffs you and waits.
You ride your horse charging into the dragon lair.
With +9 initiative, you likely win, and get first strike.
There is a small chance the dragon survives till round 2.
If you can get within 15 feet of an enemy it will be useful against and manage to hang on to concentration sure .... but that is not going to happen a lot.
60' move speed, 120' with a horse + 180'+ with a monk, +9 and advantage on concentration saves + qbsorb elements + auto-succed on concentration checks.
I prepared. Also, not the only way of doing it.
Ready a Scorching Ray and have someone use Dimension Door. For instance.
Against the Tarrasque this won't work. But that's a rare case. And you can just cast fly on the Gloomstalker and let him take care of it.