Slight exaggeration of infinite but it really could be damn close
Sentinel ... feat. (maybe a polearm mastery for a bit more)
Whenever you hit a creature with an opportunity attack, its speed drops to 0 for the rest of the turn. This stops any movement they may have been taking.
• Creatures within your reach provoke opportunity attacks even if they took the Disengage action.
• When a creature within your reach makes an attack against a target other than you (and that target doesn't have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
Okay, now I'm seeing a little more of what you are saying.
Honestly? That is just a benefit of sentinel in my mind. You can lock down single creatures the exact same way right now.
But, I would probably (after a round or two) have the creatures take the dodge action. The spell does allow them to do so if they can no longer run, and you are preventing them from running. This means you have to land the blow ad disadvantage. Still possible, but hitting all of them with disadvantage would be tough.
Plus, fear allows you to save at the end of every turn, so the sentinel could very easily be surrounded by enemies after pulling this, and start getting smacked down.
So, overall, powerful and nice use of tactics, but I wouldn't call it broken. After all, Fear can also drive an enemy to take 40d4 damage from spike growth, if you can set up the run perfectly.
@tetrasodium - regardless of difficulty, my point is more that there shouldn't even be a check and that interruption is both easy and, if successful, automatic. (e.g. if the thrown wine misses Alice entirely then she can carry on and might not even realize the wine was thrown at all*, but if it hits her then goodbye spell)
3e introduced the idea of resisting interruption, and doubled down with the combat casting feat (did any caster ever not take this?), and in so doing completely shot itself in the foot.
* - I've always had it that while casting, casters generally lose most awareness of their surroundings other than their spell's target (if it has one), as all their concentration and mental focus is going into casting the spell.
Frankly, that just seems like a good way to have people waste their time.
Sure, "That's why the fighter protected the wizard" but you can't protect them from arrows, "Well, that's why the wizard took cover" which means either A) they have no targets to hit or B) An archer just has to ready an action to shoot the wizard when they pop out of cover, and waste the spell. "Well, that's why they cast protection from arrows" So now we have a spell you need to cast to be able to cast other spells.
It just seems like a lot of effort to prevent players from doing things. I'd much rather my players did things.
Yes, but...
If goblins are known to use poisoned arrows, then you would approach potential encounters with goblins differently. There are also ways to mitigate that in game. For example, while poisoned arrows could be very effective, you can gate it behind some safeguards. For example, a critical hit. Or an attack that rolls maximum damage on the die. Or the dosage could be insufficient, granting advantage on the save unless those conditions are met.
After the first attack, where the poison is known, the players/PCs will alter their tactics. Just like they would in real life.
So, you don't want players to actually face save or die effects. You want them to fear save or die effects and when they don't react appropriately, then kill them.
Because otherwise, why would you change how poison works after they know poison is in play? Honestly, that is just so much worse to me. Player 1 gets hit, and takes damage, but is fine. Player 2 gets hit next round and dies. The Dm shrugs and says "sorry, player 1 got hit with a lower dose than you did"
That would not fly.
If certain centipedes, spiders, snakes, etc. are venomous enough to be deadly, then the people who have lived with them in their world for thousands of years will be aware of that fact. Sure, adventurers far from home might run across a lot of unknowns but they should also be treating such unknowns with caution. You can also provide some clues to the risks involved if appropriate.
Notice that in my snake example (becuase everyone knows snakes are venomous) I pointed out that knowing that might not matter. If you are thrown into a pit of snakes, you don't have a lot of other options, whether you know snakes are poisonous or not.
Traps, too. I pointed out the pits in ToH specifically because the traps were designed in such a way to make it unlikely the first person to trigger it would be unlikely to die. At which point, the presence of traps is now known and they can take appropriate actions. Traps have always been a challenge in D&D, because a lot of DMs like to use them frequently. It shouldn't really be a surprise when traps are likely to be present, and in many cases they should be obvious, set as a deterrent.
But again, this seems to miss the point. Sure, the person who fell and missed all the spikes didn't die. So now they know traps are in the area. But they don't know that a single failed save is going to kill them. And, they likely went into this expecting traps anyways. The Tomb of Horrors, if memory serves, tells the players that the tomb is a death trap full of deadly traps. It doesn't need a trap in front to remind them that there are traps in the dungeon.
In addition, traps are often mechanical devices. I have frequently had older traps either non-functioning, because mechanics have rusted, wood has dry-rotted, crossbow strings have rotted away, etc. Or poison has lost its potency, granting advantage on saves, etc.
The point is, if they know that save or die is a possibility, and they aren't sure how dangerous this particular circumstance is, you can reasonably, and believably, give them a way of knowing what the danger is that they face, so they can adjust.
When you feel the need to make it less likely the players will fail, you need to look at why that is.
If you find yourself granting advantage on saves, because a failure will kill them, and you don't want them to die. Then maybe instead of giving advantage, you should make the trap not deadly.
You are right that alice is not used to wine glasses in the face as opposed to getting stabbed with spears.
In the middle of a battlefield, a wine glass would not be a significant thing your right... but in the context of alice sitting down to negotiate with a frenemy alice is also not used to trying to stand up from the negotiating table & immediately begin gesticulating VSM components to fireball the other side in negotiations
nor is she on a battlefield.
In 3.5 alice would be casting defensively (possibly with combat caster) on the battlefield where she might otherwise get stabbed with a spear, but the 5e change of an easier save & not needing to wait till the start of her next turn means that alice is always walking around with a loaded bazooka in a firing position even while having a discussion in a civilized environment that would preclude her from doing so & now needs to be treated as such with all situations looking like either a battlefield or an execution ground with this readied at all times... that's just a disaster for world building and storytelling that encourages the players to treat everything like murderhobos or require anyone they might need to negotiate/socially interact with on not quite friendly terms to be so absurdly powerful that they are capable of acting as a solo encounter.
This causes problems within 5e itself.
For example. In 3.5 she could have gotten it off with something like quickened still silent spell at a much greater cost & in 5e subtle spell does skip somatic
or verbal but now she can literally stand in front of the bbeg & not need any of that to hit him with fireball unless he has a readied action to hit her like a freight train or counterspells her.
She only even needs subtle spell if she wants to avoid alerting people in the next room & the gm needs to fight the system or gm fiat it away.
I know I've said it before, but seriously man. Run-on sentences. Please stop with them it makes you incredibly hard to read. That entire underlined portion is a single sentence.
First, to see if I understand your 3.5 point, I looked up Combat Casting feat and Defensive Casting. Interestingly, they do not slow down your casting time whatsoever. So, Alice can stand up at the table, and cast defensively with Combat Casting. So, she can react the exact same in the negotiating room as she can in the battlefield.
In fact, if I understand casting defensively correctly, if she succeeds on that check, they don't even get to throw wine in her face, because casting defensively prevents you from taking attacks of opportunity while casting. Sure, maybe she is doing a round spell instead of a standard action spell, but you posted the rules for Fireball and it is a standard action spell, so it happens by the end of her turn. To happen by the start of her next turn, it needs to be a 1 round spell.
So, I don't even feel the need to address your points about 5e, because they apply equally well to 3.5. Concentration was a skill, one that any caster would seem to invest in, and I know skill bonuses in 3.5 could get quite large. In fact, just some quick napkin math...
Skill ranks equal level +3 max. 3rd level wizard means +6, Combat Casting is +4, assume Con of 14 for another +2. DC to Defensively Cast Fireball is an 18, you have a +12. That is a 75% chance of not even dealing with the wineglass, and another 70% to 85% chance of succeeding after that.
And this is at 3rd level, a 5e wizard can't even attempt this until 5th level, because they don't get Fireball until then. And at 5th level, Alice in 3.5 has higher skill ranks, so her chances of succeeding are even higher.
Even if we give 5e Alice Warcaster, not being proficient in Con save and with the same stats, gives her about an 85% chance of succeeding. The exact same.
So, any problems you see in 5e with this scenario are equally present in 3.5.