A smart party would, as soon as the fog arrived, run out of the fog cloud and surround it in order to pick off any enemies that escape, while leaving the crab on its own to take care of business inside the fog.
True, but this is still the same criticism that comes to Darkness-Devil Sight. You are limiting vision not only for the enemy, but for your side as well.
And, abandoning the battle and leaving it to a 6 to 8 damage per round crab.... that is going to be a drag.
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Giant Badger's Burrow Speed is slow, but it only needs to move however long and then burrow for the last 5-10 ft to be out of sight. In fact, attack-burrow round 1 then burrow-attack-burrow could possibly be a great pattern, especially for enemies with multiattack or when you're level 7 and can have them BA disengage (no attacking, though). 8 damage is better than what base ranger can do with a bow and arrow until level 4 but outpaces again at level 5. The to-hit is similar unless you take archery style which you very well might (though, defense isn't bad either). Plus, you can attack from over 150ft and not have disadvantage.
Even if you assume that burrowing down 5ft when you fill a 5 ft cube makes you immune to attacks (which is not guaranteed) you still get hit with attacks of opportunity, meaning that until 7th level you are moving up, attacking, then giving them a free attack. With bad AC and bad HP... this is a bad plan.
Also... really? Base Ranger with bow and arrow at level 3 is potentially doing 1d8+3, or 7.5 on average. Right there with the Badger.... except that any level 3 ranger who is not a Beastmaster has another subclass, and almost all of them offer a way to add a 1d8 or 1d6 to their damage.... and you can potentially use Hunter's Mark witha bow and arrow, which you can't with a Badger, meaning that you are looking at closer to 2d8+1d6+3 or 15.5, nearly double the badger's output.
Also, while, yes you can technically command your badger to move over 150 ft to attack at a distance, this likely takes at least 6 turns, meaning that this only works for ambushes, and if you are ambushing more than a single target.... dead badger. Additionally, you give verbal commands, meaning that the badger has to be able to hear you, so you can't command it to attack by havinging it burrow up to the enemies, because under ten ft of dirt, it can't hear you. (All of this assumes RAW, which I acknowledge, RAW is stupid)
Until you notice the larger blindsight which pulls into the strategy I've mentioned and will go into greater detail in later.
Their stealth, as a companion, is +9, which is higher than a Ranger could possibly get at that level save for Pass Without Trace, which the spider also benefits from.
The Wolf Spider should be looked at post-buff and with everything they have. Pre-buff spiders have nothing to do with the discussion. The Wolf Spider Ranger can deal 23.5 if they don't save and 20.5 if they do save.
How the actual heck do you imagine those numbers happening?
Assuming a +6 bonus to the damage, which is base 1d6+1 and 2d6 that only gets me to 17.5. Which tells me that you are double dipping into the ranger bonus, and applying it to the piercing and to the poison, which is not how those bonuses work. Or, maybe you are counting both attacks, which gets me 2d6+14+4d6= 35... so no, you are applying to damage boost twice, which is not how the ability works.
So, 17.5 for a single attack, 35 for a double. Not bad... except that 4d6 of that (14) is coming from poison damage, the most common immunity in the game, so we can assume that against a lot of high level foes you are actually dealing 10.5 and 21 damage.
And, that requires being 17th level at a minimum, and by that point the ranger should have at least a +3 bow, right? So let's compare the Hunter Ranger. They would deal 3d8+1d6+16= 33 damage I'm only two points behind the spider who gets two failed con saves from something not immune to poison at level 17. By doing nothing except the same bog-standard approach I had at level 5.
Oh, and let us not forget that by level 17 your spider would have 68 hp. An Adult Red dragon is a CR 17 fight. It does an average of 58 damage a round. So, high likelyhood of it dying in a single round of combat at that level.
It'll probably die, but just get a new or different creature if it does. This is also one of, if not THE most reliable ways to restrain someone at early levels. The 6 damage is excusable but it can also swallow small enemies (which might not be all that uncommon in typical play cough*goblins*cough.
And this is one big problem I have with these rangers. You are going in expecting them to die early and often, and you just keep getting a new one each day. That is not the class fantasy of this archetype.
They take 13 damage if they fail. Also, they don't actually have to fail the DC 12 strength check to be BA attacked, they just have to be prone. If the fighter decides to prone the enemy, the Panther can capitalize. If the Wizard uses a spell that forces the enemy prone, the Panther can capitalize.
So, if the fighter wastes on of their attacks to shove prone, then the panther can deal an extra 7 damage... less than the fighter likely would have done by just using their attack.
Sure, there are ways to have a party set-up where that happens, but a plan that involves that much help from other characters is not reliable. That should be a bonus on top of what you can do, not your go to plan.
3rd level is when you get the subclass, though.
Here's the cool part, hehe.
You and your party don't get disadvantage when you cast fog cloud on your enemy. You can't see your enemy, true, but they can't see you. All attack rolls are neutral at this point. All attack rolls except for the ones between the crab and enemy. The crab can see the enemy, so they are given advantage while the enemy has disadvantage against them.
This is a hotly contested and often houseruled interpretation, especially since it makes those spells and effects that use heavy obscurement useless. So, big ol' "Subject to DM approval" sign right here.
(At my own tables, we have ruled that if everyone is blind, everyone has disadvantage)
Ready for the super-duper fun part? Hehehe, here we go.
If you actually put the edge of Fog Cloud targetting the grappling Crab but not the enemy, you get exactly the same situation except that the enemy has disadvantage on everyone while everyone else has neutral attacks. I'll give anyone a heart eyes reaction if they can tell me why this happens, its not too hard, though.
Huh?
If the edge of the fog cloud is on the crab, but not the enemy? So, you want to position this cloud so the crab is obscured and nothing else is? That does nothing to the enemy attacks against anyone except the crab. Unless you are planning on stepping in and out of the fog cloud?
Which, okay, fine, you have a small possibility of diving in and out of the Fog Cloud and making it hard for this one enemy... until they ready an attack to hit you as soon as you exit the cloud. Those getting you when you aren't in the cloud. Though, I'd probably just work on killing the crab and hit you with an attack of opportunity when you left. Since that is two attacks instead of one.
Also, if you have a single enemy vs the entire party, you are in bad news territory for the enemy anyways. You have to be well above the parties CR for that to be a challenge typically.
And if you're level 7, you can order the crab to help as a bonus action and turn up the damage by alot (maybe the Paladin can put some extra oomph in their hit or the Barbarian doesn't need to go reckless this turn).
I love this. Because it is just so... I don't quite know.
See, at 7th level you have gotten the privilege on using your bonus action on your turn to give the help action.
At first level my Human ranger could take Ritual Caster and get a familiar who can do the exact same thing for free. Or I could pick it up at 4th or 8th. Or I could just see if the party already has someone with that spell. And then I can just be a different subclass.