Rangers Don't Get Foci
All classes that use a focus have a "Spellcasting Focus" subheading to their Spellcasting (or Warlock) feature:
- Wizard (arcane focus)
- Warlock (arcane focus)
- Sorcerer (arcane focus)
- Paladin (holy symbol)
- Druid (druidic focus)
- Cleric (holy symbol)
- Bard (musical instrument)
The wizards feature for instance reads:
SPELLCASTING FOCUS
You can use an arcane focus (found in chapter 5) as a
spellcasting focus for your wizard spells.
PHB 114
With the exception of the Ranger and Eldritch Knight, which specifically don't mention foci as part of the spell casting feature. Who have no such text.
It Is Intentional
Jeremy Crawford clarified in a tweet that the omission of focus for the ranger class is intentional. He was asked:
[D]o rangers use spellcasting foci, and/or do they need to buy component pouches at 2nd level?
And replied:
The ranger doesn't have a spellcasting focus. The trusty component pouch will do the job.
https://twitter.com/JeremyECrawford/status/687417277231267844?ref_src=twsrc%5Etfw
This makes sense as most rangers would likely have a bow. All bows, even the hand cross bow (even with Crossbow Expert), requires a free hand to load. So holding a focus would mess with the traditional ranger with bow. You keep a free hand for spells, pouch and loading firing arrows.
Mechanically
Using a bow, you have the weapon in one hand and the other on string. After you fire, your hand is free again. When you cast you pull out the components and put them back, or they are consumed, and your hand is free again. Works great with a bow. However, the arcane and druidic focus require object iterations, and you either have to drop it or spend the book keeping at the start and end of each turn.
Yes, a holy symbol could be worn and keep the hands free, likely why they did that for paladins, but they chose to go a different direction with the ranger.
Thematically
Ranger forage for food and materials, they are self reliant. It isn't hard to see them restocking their component pouch as they move through the wild area.
What about the Mistletoe?
it might be impossible to find mistletoe in the desert when a material component is needed.
The PHB says this about the component pouch:
Component Pouch. A component pouch is a small, watertight leather belt pouch that has compartments to hold all the material components and other special items you need to cast your spells, except for those components that have a specific cost (as indicated in a spell's description).
PHB 151
The pouch has all the components your spells require, and Material Components aren't consumed unless the spell says they are:
If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell.
PHB 203
So, you have mistletoe once, and you have it forever -- unless your DM rules that lose it or it goes bad, etc. Then you have to buy more mistletoe or buy a new pouch. Such a thing, however, isn't in the rule.
Unearthed Arcana / Tasha's Cauldron of Everything
A recent released play test (UA), and page 57 of Tasha's Cauldron of Everything (optional rule) provides the following to the Ranger class:
Spellcasting Focus
2nd-level ranger feature (enhances Spellcasting)
You can use a druidic focus as a spellcasting focus for your ranger spells. See chapter 5, “Equipment,” of the Player’s Handbook for a list of things that count as druidic focuses.
As written, the ranger gains advantage on all initiative rolls and on (most) first-round attacks.
The relevant portion of the class feature Natural Explorer is simple:
- You have advantage on initiative rolls.
- On your first turn during combat, you have advantage on attack rolls against creatures that have not yet acted.
You have advantage on initiative rolls. Is it an initiative roll? Then you've got advantage.
You're attacking someone? Did they act yet? No? You've got advantage on that attack.
This clearly contradicts the intent of the class. Or does it...?
The opening line of Natural Explorer sets the tone:
You are a master of navigating the natural world, and you react with swift and decisive action when attacked.
It seems like you're reading onto that line the proviso "react with swift and decisive action when attacked in the natural world." But it's not there in the text. (To be fair, that's the read I gave it at first, too.)
Most of the features of the new ranger do tie into the nature theme. But this isn't the only that's just a pure buff to combat: Fleet of Foot, Vanish, and Feral Senses all would apply (as written) equally-well in an urban or dungeon environment as in overland.
The intro to that UA clearly states that the class--both as a whole and its features considered severally--was just seen as weak and unfun. It's your call as to whether they've gone too far pumping some power and fun into the Ranger, but that's exactly what the authors were trying to do.
Best Answer
It depends on the setting.
In some Dnd settings the Underdark is more than just a cave or an underground passage. It's a massive underground network that can potentially span an entire continent. It's kind of like another layer of the world.
In other settings this layer may not exist, or be a lot smaller in scope. In this case it's up to your DM to decide what defines Underdark. It could be any underground area, an especially deep/dark underground area or a specific layer as above.
In my current homebrew campaign setting I just define it as any large natural underground area. This brings it more inline with the other terrain types (Arctic, Coast, Desert, Forest, Grassland, Mountain, Swamp) and also means it is more likely to be useful for the ranger, as the party journeying to the Underdark proper is unlikely.