Here is the scenario:
I am attacking a fighter with a crossbow. I run out of bolts and am unarmed as he charges at me with a spear. My opponent has a short sword on his belt. He misses with his initial spear attack and it's my turn.
I want to take his sword from its sheath and, if possible, attack him with it.
What process or series of ability/skill checks need to be made for this to work?
I'm considering a scenario where I don't have any class specific features that would change the process.
For every ability check, the GM decides which of the six abilities is relevant to the task at hand and the difficulty of the task.
So first and foremost the decision is up to the DM.
The description for Sleight of hand says:
Sleight of Hand. Whenever you attempt an act of legerdemain or manual trickery, such as planting something on someone else or concealing an object on your person, make a Dexterity (Sleight of Hand) check. The GM might also call for a Dexterity (Sleight of Hand) check to determine whether you can lift a coin purse off another person or slip something out of another person's pocket.
It's not a big stretch to say pulling a sword out of someone's scabbard fits the criteria. But given that you are in combat, this would most definitely be a contested skill check, and as a DM, I'd say the contesting skill (used by the sword's owner) could either be Athletics or Acrobatics (same as grapple checks) since you're not trying to sneakily take his sword (where perception would be the contesting skill)
And here we get into the action economy.
Normally, grabbing the sword would cost an action. So there goes your chance to attack with your newly acquired shortsword that round.
However, certain class abilities like Action Surge (or potentially a grabbing the sword as part of a bonus action available to your class) might give you the opportunity to still attack with the weapon on the same round you acquired it.
This is exactly the kind of thing I would want to see if I were your DM. I would let you do something as a reaction using The Rule of Cool (or Rule Zero):
You are essentially trying to "pickpocket" your opponent which the book describes under Sleight of Hand.
However, in this case, I think it would be a straight Dex check and not Sleight of Hand because the book says that is what you do when you're trying to do "legerdemain" which entails doing something without being noticed.
You have the full attention of an opponent that is charging you so I think the added skill of Sleight of Hand trying to do it deceptively wouldn't apply but, because I'd want to see this happen as a DM, I wouldn't protest it if the player pushed for it.
Also, I don't think it's a strength check unless the sword is somehow buckled or strapped into its sheath. You're really just seeing if, in the flurry of the moment, you can grab the handle and yank it free without the opponent stopping you.
In any case, rather than set a fixed DC, I think this would fall under the rules for Contests in Combat where opponents make opposed checks.
From page 271 of the DMG:
A creature can use a weapon attack to knock a weapon or another item from a target's grasp. The attacker makes an attack roll contested by the target's Strength (Athletics) check or Dexterity (Acrobatics) check. If the attacker wins the contest, the attack causes no damage or other ill effect, but the defender drops the item.The attacker has disadvantage on its attack roll if the target is holding the item with two or more hands. The target has advantage on its ability check if it is larger than the attacking creature, or disadvantage if it is smaller.
I don't think it applies since the opponent isn't holding the weapon but I also don't think it's entirely unreasonable to use the rule in this situation either.
Since the opponent will likely clamp their hand down to try and stop you (thereby sort of grasping it), you might treat the sheathed sword as being very roughly the equivalent of being held with two hands (thereby applying disadvantage to your attack).
Here's what I would do if you tried to do that in an Adventurer's League game I was GMing:
"I grab his sword from its sheath and attack him with it", you say.
"Are you a battle master or something? Do you have a feature that lets you do that?", I reply.
"No I was just thinking he's close so I should be able to grab at it" "Sorry, there's no action available to you that lets you do that. Stuff on people isn't really something you can typically interact with during combat, unfortunately. You can [list of options available to player]")
Here's what I would do if you were playing in a more improvisational game with an emphasis on detail-oriented combat:
"As I dodge past his spear into his reach, I make a grab his belt, aiming to free the sword from its sheathe", you say.
"He swings up at your face with the butt of his spear, rotating his hips to face you square on, which will make reaching the weapon difficult, even should he miss", I reply.
I roll an attack roll with the better of dexterity or strength for the fighter, adding proficiency only if I think he's particularly good with spears. His reaction is used for this, sort of like an Opportunity attack. If he hits you, he deals 1 to 1d4 damage, depending on how I'm feeling and what level you are, and I narrate him keeping you back-- you don't get a roll (that's what your AC is representing here). If he fails to hit you, then you get further inside his reach and you get to roll a Dex check to get his sword off him. It's a really hard check, so it's gonna be a DC 15 or 20 depending on how I'm feeling. Various proficiencies, like Sleight of Hand for example, may apply. If you get like a 30 you can fling the sheath in his face while you draw it and give him disadvantage on his next thing he does, but there's no way I'm letting you attack with his sword while grabbing it off him as part of the same action. If you succeed, you guys swap places on the battle mat, if you fail, you're on what used to be his left.
Here's what I would do if we were playing an intrigue-focused game I was GMing:
"I grab his sword from its sheath and attack him with it", you say.
"Yeah, but you don't though", I reply
"What?" You ask, confused. "He's a professional guard. He's been training every day for the last 10 years, and regularly deals with desperate criminals making last-ditch escape attempts. He's in the prime of his life. You are a 60-year-old court ambassador slash international spy. You know both that trying to do that will all-but-definitely get you killed and how to keep a cool head under pressure and that even if by some miracle you did get the sword away from him he'd still beat you in a straight fight. Try something else."
Here's what I would do if it were a superhero-tier game and I was GMing and your opponent was a random dude:
"I grab his sword from its sheath and attack him with it," you say.
"Roll an attack v.s. AC 16 with disadvantage," I reply. "Cool, I hit," you say.
"Nice," I respond, not bothering to have you roll damage, "now you have a sword and a dead fighter. What do you do next?"
As you can see, both the difficulty of the task and whether or not it is possible varies wildly between these examples. And that's just how I have run this sort of thing-- the variance will be even larger if some other person were involved as GM. The rules provide a framework you could use to let people attempt these kinds of things in a wide variety of different ways should you so choose, but whether or not to even let an attempt be made in the first place is also a question the DM must answer and on which the rules don't really have much guidance.
