Contest should always happen for achieving control over something. in the example you talk about grappling and pushing is freedom of movement or space. Another example could be for example someone blocking a door while someone is trying to open it.
Sometimes one character’s or monster’s efforts are directly opposed to another’s....This situation also applies when one of them is trying to prevent the other
one from accomplishing a goal—for example, when a monster tries to force open a door that an adventurer is holding closed. (Basic D&D p58)
It could never be a direct attack so for example it could never be
- setting someone on fire with a lamp (it's an attack , you crash it on her)
- blinding someone with sand ( that's also an attack )
- flipping a table in the face of someone ( attack )
- dragging the carpet under the feet of someone ( attack )
Sand , tables, carpets are all improvised weapons that could do no damage but they could cause effects or trigger an opposed roll. At least is how I see it.
Now situational bonuses or penalties do apply, for example a lamp can be used as a flail but it's not one, so, a penalty to attack and damage should apply. Sand is not designed as a weapon so it doesn't count as a throwing dagger. Each case is completely different from the other, you cant forecast what the players might think in their desperation or just for having fun and it's not worthing to spent time designing improvised weapons. Just tackle it as it happens, agree with your player's at the spot with a formula that will apply for that moment only. What matters is for the improvised attack to contribute to fun, by being just as hard and rewarding to avoid letting your players using this solution as a loophole especially if looks temptingly effective. Make things interesting by raising the stakes, increase effect and difficulty (made up some reason) or add a backfire case if the attack fails.
I don't see any reason though why not using blindness as the condition when for example you wrestle with someone to pick wis eyes out or fight with someone to extinguish the only source of light in the room. But in this case you actually fight over the torch, Blindness is a side affect because there is no light in the room anymore.
Good examples of contests are
- Fighting over a light source.
- Wrestling to grab the one ring from the floor (Golum vs Frodo)
- Trying to Swim while someone is dragging you down. (You both truing to reach the surface while trying to kill each other at the same time)
- Trying to disarm someone by means of grappling. (You both trying to hold on the weapon)
- Playing Pink Pong or Tennis :)
- Drinking Contest
In any case, Contest is a hack to the rules in order to cover some unexpected cases when it's not covered by the existing rules and usually the effect is more of a strangle towards something. How exactly you are going to deliver that, is what you have to improvise.
Again as with the improvised attacks, there is no rule that tells you that someone is not going to wrestle to blindness, to castration or to death. (All that actually happened in ancient olympics btw)
Anything that involves strangle while doing the same thing or opposing over the same thing is a valid Contest. Anything that is actually like "I'll grab the carcass of the giant frog and I will brake your head" is an improvised weapon attack.
Contests tent to be more abstract and Improvised attacks more specific. Both are abstract rules for actions that are not ruled by the book and it doesn't really worth in most cases to worry about. by experience you make rules of the thumb and you keep the game going.
To conclude
You can have whatever effect you like with your improvised action as long as it makes sense.
- Use improvised action and Contests for more generic actions.
- Use improvised weapon when you have an improvised implement.
- Make things interesting cooking the difficulty/effect relation
P.S. SevenSidedDice mentioned a few examples and over that we disagreed. The first one is if grappling is an attack. Yes it is and instead of unarmed attack vs AC : resolves to dmg it's actually STR vs STR : resolve to grappled.
Trying to blind someone by throwing sand though should be an attack, using sand as an improvised weapon. Because the implement of the action is in your hand.
Using the cloak of the opponent to blind him... well the item belongs to the opponent, you have to take control of it first, then you have to move it to her face to cause a blinding temporary effect. There are many ways of doing this but two of the could be. Either you simplify it with an improvised action DEX vs DEX. Or spit it to two actions, a sleight of hand to grab the cloak and an improvised weapon to actually blind the opponent with her own cloak.
So, while improvising actions, you can choose between using improvised weapons or improvised actions or mixing both depending on the context.
Interacting with an unwilling targets items, I would considered an attack. but again whatever makes sense depending on the context.
Going off the phrasing of the effect
When the Lich takes damage
Not "When the Lich is hit with an attack/spell/etc," when they take damage.
In this case, a Lich's resistances and immunities are applied before you determine whether or not Tether fires off. Because until you apply those, the Lich hasn't taken any damage yet.
So, since when you hit a Lich with a nonmagical sword they take Zero Damage, then even if you still ruled that Tether activated, then the Wizard would have to save against taking half of zero damage...so, no damage at all.
The specific wording you are referring to of
Resistance and then vulnerability are applied after all other modifiers to damage
Are all still applied before a creature actually takes any damage. It's not like a Fire-immune creature gets hit with a fireball, takes the fire damage, and them immediately heals that damage. They simply don't take any damage at all.
Best Answer
It will generally work, but leans into a highly technical, meta-game style of play. This is not an advantage for players.
First, the basics: destroying line-of-sight, by any means, is generally going to ruin spells which require line-of-sight to cast. Getting an opaque sack with a sufficiently tight weave (an important consideration your players might not consider in advance) over an enemy's vision organs will destroy line-of-sight. So would throwing up a curtain in front of yourself and moving back one space. It works, according to a strict interpretation of the rules.
But it works because of the rules rather than because of any obvious, innate rationale in-game. It's like a long rest healing virtually all injuries: if the rules did not specifically describe that effect, no one would try to use it that way.
It shouldn't be easy. If you decide to allow such a tactic at your table (and you would be well within your authority to forbid it), it should not be an easy-win strategy. Getting the sack onto the lich's head should involve an expenditure of resources (even if it's just time or surprise) and risk of failure with attendant consequences.
If they succeed they get some advantage by limiting what spells the lich can use for some amount of time. That's not unreasonable, as they took some risks to gain that advantage and faced a risk of being worse off if they had failed (even if the consequence is just wasted actions).
It shouldn't be unexpected. Liches are, by definition, not stupid. They are also highly accomplished scholars of magic and are avid magic users. Together, these indicate that a lich will be 100% aware of the possibility and consequences of having their vision affected like this. You would be absolutely justified, in-game, to have the lich be specially aware of this danger (making it harder to accomplish) as well as have some contingency plans to deal with it (making it less valuable even if players succeed). A Legendary Action casting of Mage Hand immediately after the sacking is enough to deal with this, for example.
It's doesn't make the lich easy to deal with. Liches have access to lots of spells, many of which don't require line-of-sight. Some of them might be particularly dangerous for the PC tasked with applying the sack as well. Thunderwave is bad news, and Cloudkill is enough to ruin your day if you'd imagined yourself safe from magic spells.
D&D is a game. Cleverness is great, and I always like to reward it at my table. But the point of a TTRPG is not to trivialize encounters with an "I win" button. Even if everything in the rules lines up to suggest that some tactic should always be effective, if it ruins the game then the DM's job is to change things up. If all the players want is a way to tip the scales in their favor against a lich, you can make up a quest to recover the MacGuffin Sack of Lich Blinding that does so. No amount of clever rule dissection should allow players to skip the game itself.
Wise players should fear this sort of rules-wrangling. Even if some (or all!) of your players are strong experts on the rules, the DM's control of the game world makes "technically-this-works"-style strategies a bad route for them to take. If my players want to split hairs with me for some hyper-specific scenario, I'm very likely to grant whatever it is they're trying to achieve. And then adjust the game world and contents to utterly savage them. They tend not to understand the rules as thoroughly as I do (so my hair splitting tends to hurt them more than their hair splitting benefited them, and more often), and I am totally unconstrained from presenting situations which specifically frustrate their meta-derived tactics.
Players tend to be attracted to tactics like this
At least my players seem to have a real passion for it. My typical approach to this, which I have explicitly explained to my players, is that clever plans and cute ideas certainly can work. As long as I haven't informed them that some aspect of the plan is forbidden they can rely on things that should technically work to actually work.
For a time.
Coming up with a clever tactic or novel solution to a problem is, in my mind, most of what these games are really about. In that spirit, they can get a nice benefit from a tactic that I consider to be cheesing the game at least the first time they pull it off. And maybe more times after it. But, like nacho dip at a party, the cheese just doesn't last very long.
I vary the setup to suit the narrative, but the advantages don't persist forever. Enemy organizations learn about the tactic and eventually respond in a way that nerfs the advantage. Or I design combats where the tactic is less effective or impossible to carry out.
My players have responded well to this. They like it more than simply having their idea banned. And there is some element of resource management to it: if they try the tactic every time they can, then they are a lot more likely to encounter situations where it doesn't work as they intend. It goes from a sure-fire approach to an idea good enough to try, but not to rely on too strongly. If they are a bit more reserved and only use the tactic from time to time they can be a lot more secure in thinking that it will tip the balance when they need it.
What I would do here
I've never had this exact situation, with a sack and a lich, but my players try things like this all the time. I try to funnel considerations about how well the tactic should work, rules aside, into existing game mechanics. So in this case I would lay out two broad options.
If they want to try to throw the sack onto the lich's head, I would treat the sack as an improvised weapon. This makes an attack roll harder to succeed with, and also provides a nice opportunity to get some benefit out of the Tavern Brawler feat (which I always seem to have one player take). I would also be very strict with the throwing range of the sack and with inventory management (once you throw the sack, you'll have to go pick it up if you want to try again).
If they want to jam the sack onto the lich's head manually, I would treat doing so as an opposed grapple, and almost certainly with Advantage for the lich. The lich is not going to just sit there and let the players run through whatever it is they want to do, and as above they would be very aware of this tactic long before the PCs ever tried it.
In either case, the sack is not going to be in place for very long. The lich can easily remove it themselves through a variety of means which don't require an action. The lich can also move out of the area affected by Silence pretty easily, and even if it can't has options that will make that spell hard to maintain (Paralyzing Touch and Disrupt Life are both problematic and available off-turn as Legendary Actions, for instance). I would also be extremely likely to give the lich a minion or two, making this strategy even harder to pull off and even less reliable.
In no case would I set up a combat encounter with a lich and allow the PCs to have a "super easy" fight with such a no-investment tactic.