if i am playing an yuan-ti character/ some homebrew race with poison immunity or i somehow come across an magic item that gives out passive poison immunity, what new strategies will start to open up because of that? obiously first instinct is to use stinking cloud or cloudkill both as an stealth tool that makes an area heavily obscured and as an shield that prevents melee enemies from getting close without being victim to the spell while also making me heavily obscured for any archers who try to shoot me with arrows, but is there anything else i should consider? any spell that i suddenly become immune to? any strategies i could use with this immunity that i could not before?

Chuck a vial of acid at your feet, surrounded by people. 

Quote from BramblefootDruid >>

Chuck a vial of acid at your feet, surrounded by people. 

 ...poison immunity does not give you acid immunity, also vials of acid do not deal splash damage for some reason so by doing so you are literally just shooting yourself in the foot, there is not benefit to doing that you are just gonna hurt yourself and then possibly accidentally step on the shattered glass oh yeah and also a single acid vial cost 50 gold pieces like this is just paying a large sum of money to deal 2d6 damage to yourself in the middle of combat do not do this

its not a very exciting thing to be immune to. dwarves and warforged are resistant, yaun-ti and grunges are immune.  protection from poison is only a 2nd level spell. antitoxin is a mundane item in most stores - so there's not a lot out there that hinges on poison damage, its more like a 'oh yeah, poison damage too'.

if it were to come up in a game, i'd allow a character to hold their breathe with stinking cloud as long as they weren't surprised and could see it coming since the spell specifically discusses breathing (don't think i'd allow that for cloudkill though - maybe its a contact poision).   i also don't know how effective they'd be as a stealth tool...giant cloud of yellow noxious gas is kind of a 'hey everyone, look over there' type of thing.   ...so maybe you could get past someone with cloudkill without them able to hit you, but they'd definitely know you're there.

Being able to trick NPCs into eating/drinking poison by sharing a cup with them is the first thing that springs to mind. In combat, immunity to Stinking Cloud and Cloudkill like you mention sounds great, though I'm not sure there are a ton of other Poison damage/condition spells that you are likely to need to wade through. Xanathar's mentions indirectly that there may normally be a "risk of exposing yourself to its effects" when applying poison, maybe also when trying to harvest monster's corpses for poison, I suppose you could ignore that. You might be able to make the argument to your DM that poison immunity without disease immunity should allow you to become an asymptomatic carrier of diseases (able to spread them, but not suffering any ill effects from them), but that's going beyond RAW and your mileage may vary. 

Quote from Chicken_Champ >>

Being able to trick NPCs into eating/drinking poison by sharing a cup with them is the first thing that springs to mind. In combat, immunity to Stinking Cloud and Cloudkill like you mention sounds great, though I'm not sure there are a ton of other Poison damage/condition spells that you are likely to need to wade through. Xanathar's mentions indirectly that there may normally be a "risk of exposing yourself to its effects" when applying poison, maybe also when trying to harvest monster's corpses for poison, I suppose you could ignore that. You might be able to make the argument to your DM that poison immunity without disease immunity should allow you to become an asymptomatic carrier of diseases (able to spread them, but not suffering any ill effects from them), but that's going beyond RAW and your mileage may vary. 

 i thought about that eating/drinking thing too...but then i thought there's no way i'd ever assume something is safe after seeing a yaun-ti drink it.  no way.  the poison harvesting idea is pretty good if your campaign is into creature harvesting and you want to make poisons (or even just sell the materials), but that's more story/RP than combat.

maybe grapple with contact poison on your body - like an impromptu grung.  ...using damage-dealing poison would kind of defeat the purpose of grappling though as it'd probably easier to just stab them.   ...so not sure - maybe some other kind of contact poison, like paralyzing or something.

The 5E race description doesn't emphasize it like prior editions did, but Purebloods are supposed to outwardly be indistinguishable from regular humans (at least, when they're undercover and taking measures to conceal any minor serpentine mutations they may have). The race description still gives a bit of a nod at this, notwithstanding the very-snakey looking portrait they slapped on the page:

Only purebloods — which walk among humanoids and therefore have to learn how to speak to them civilly — practice interacting with meat-creatures. Much of their training involves suppressing their innate annoyance at having to speak to lesser beings as though they were equals, or being obliged to kowtow to a humanoid ruler as if the pureblood were merely an advisor. Pureblood spies feel a sort of aloof contempt toward meat-creatures, but they can affect a pleasant tone, and speak to such creatures with a silver tongue that disguises their true feelings.

Your mileage may vary depending on your DM, but every time I play a Yuan Ti, I don't actually tell my group or NPCs that I'm not Human. 

Chicken_Champ, you are incorrect.  in 5e, Yuan Ti are NOT outwardly indistinguisable from regular humans:

Yuan-ti Pureblood

 

MM p310

Purebloods form the lowest caste of yuan-ti society. They closely resemble humans, yet a pureblood can't pass for human under close scrutiny because there's always some hint of its true nature, such as scaly patches of skin, serpentine eyes, pointed teeth, or a forked tongue. Wearing cloaks and cowls, they masquerade as humans and infiltrate civilized lands to gather information, kidnap prisoners for interrogation and sacrifice, and trade with anyone who has something that can further their myriad plots.

"Can't pas under close scrutiny" does not mean "cannot pass with efforts made on a disguise," which is why right in the quoted excerpt it tells you that they "masquerade as humans and infiltrate civilized lands." Yuan-Ti can and do pass themselves off as Human, it's literally the entire point of the perpetuation of the Pureblood cast. Not disagreeing that 5E doesn't devote as much page space to it as prior editions did, but I don't think the physiology and mindset of the Yuan-ti have changed from prior editions where they were described thus:

Serpent Kingdoms, p. 190

Yuan-ti that can pass for humans with suitable clothing, cosmetics, and magic are known as purebloods. These creatures are usually charged with infiltrating humanoid societies and managing covert operations that require direct contact with humanoids. Pureblood travel throughout Faerûn, infiltrating human societies everywhere. Most can easily disguise their snakelike features to pass for humans. Typically, purebloods serve as spies and agents, carrying out the plots of higher-ranking yuan-ti. Some, however, are ambitious enough to make their own plans, and a few even leave their fellow yuan-ti and embrace human society completely. Such purebloods can make enjoyable player characters.

Playing a Yuan-Ti Pureblood as a Human is one of the only uses I've ever actually found for Disguise Kit proficiency that Disguise Self can't match, and I heartily encourage it! 

Interesting.

Well, it depends on the DM, but in my experience DMs have treated excessive drinking as being a form of poison. So you could use your poison immunity to be absolutely beast at drinking contests.

Also question: let us say that we have a flask of unidentified poison, and i decide to take a sip to see if it is a poison i have tasted before, what kind of check would that be? Wisdom (perception), intelegence (investigation), intelegence (poisoner's kit), intelegence (nature), something else?

For someone that is not immune, I would say that taking a sip of poison should be a Wisdom (stupid way to die) check, DC 10 to not sip it.   If they did sip it, I would describe the results to them, not give them a name for the poison.    DC cut by -5, duration and HP damage cut in half.

But someone immune to poison should not be sipping the poison, even if they were immune.   Good way to use up a dose, poison already is small enough to coat a needle.

I would allow either an Intelligence (Poison Kit) check or a Wisdom (Herbalist) check to figure out what kind of poison it is.  They both would involve looking, smelling,  and dropping a small amount of acid and/or blood into the poison.

Quote from Mog_Dracov >>

Taking a sip of poison should be a Wisdom (stupid way to die) check, DC 10 to not sip it.   If they were foolish enough to do that, I would describe the results to them, not give them a name for the poison.    DC cut by -5, duration and HP damage cut in half.

I would allow EITHER an Intelligence (Poison Kit) check or a Wisdom (Herbalist) check to figure out what kind of poison it is.  They both would involve looking, smelling,  and spilling a bit of the poison and adding things like acid and blood to it to see what happens.

None of them would involve tasting it.

 If the character is immune to poisons, what is the harm in tasting it?

you won’t accidentally poison yourself while handling poisons. I might be overly harsh, but I give a decent chance for someone to hurt themselves with poison blades, arrows, and such (also things like breaking potions when they fall). It greatly ups the value of these types of buffs in my campaign. Not everybody’s cup of tea as it’s another thing to keep an eye out for. My group wanted to use critical fail/success table rules (bad or interesting things happen on all d20 1’s rolled, beneficial things on 20’s, whether or not the roll is successful). This ties in nicely. 

It's a little awkward figuring out how Tool proficiencies are "useful" for a character, which is probably why XGtE felt the need to expand on them. If I already have Investigation as a proficiency, what motivation is there to ever pull out my Poison Kit to do forensic analysis, if all I'm doing is swapping one proficiency with another? One solution would be to give the character Advantage when making a check that they are proficient in both the Tool and a relevant underlying skill.... or you could double up the proficiency by handing out temporary Expertise in the check... or you could look to XGtE for other misc. bonuses that tool proficiency might carry with it.

It should be pointed out that Tool proficiencies don't have a specific attribute that they key off of, and depending on how you're using them, several stats may be relevant. Dex (Poison Kit) to apply poison to an intricate trap mechanism, Int (Poison Kit) to brew a poison or develop an antidote, Wis (Poison Kit) to suss out the subtle nutty aromas (or even flavors, with Poison Immunity) that distinguish Poison X from Poison Y... DMs can and should be open to players applying their Tool proficiency to more than one attribute (in fact, they should be open to the same with skills as well, but even moreso with tools).

Quote from TransmorpherDDS >>

Well, it depends on the DM, but in my experience DMs have treated excessive drinking as being a form of poison. So you could use your poison immunity to be absolutely beast at drinking contests.

 have done this. veeeerrry satisfying when you challenge your party to a drink off

Quote from Chicken_Champ >>

It's a little awkward figuring out how Tool proficiencies are "useful" for a character, which is probably why XGtE felt the need to expand on them. If I already have Investigation as a proficiency, what motivation is there to ever pull out my Poison Kit to do forensic analysis, if all I'm doing is swapping one proficiency with another? One solution would be to give the character Advantage when making a check that they are proficient in both the Tool and a relevant underlying skill.... or you could double up the proficiency by handing out temporary Expertise in the check... or you could look to XGtE for other misc. bonuses that tool proficiency might carry with it.

You could also change the DC or alter the information learned based on whether Investigate or Poisoner's Kit proficiency was used- it might be a DC 20 check with simple Investigate but only 15 if you use the tools in a Poisoner's Kit, or the kit might reveal that this type of poison loses potency rapidly when exposed to air so the victim was poisoned no more than an hour ago...

Stuff like that.