So what creatures (save for Phase Spiders) exist in the Ethereal Plane and can sever the cord? I've been looking around and can't really find anything that can.

This spell actually deals with the astral plane which is a bit different from the ethereal plane. Githyanki knights, githyanki supreme commanders, and astral dreadnoughts are some monsters whose text specifically states they can sever silver cords. 

Astral dragons can bite your cord resulting in immediate death.

so what happens if you physical form dies while you are in the astral plane? Does your soul behave like Magic Jar or do you just die?

I THINK it would be like a magic jar. I don't have a source on me right now but I feel like I've heard that before.

Quote from Necropolis379 >>

so what happens if you physical form dies while you are in the astral plane? Does your soul behave like Magic Jar or do you just die?

 Well your cord would be severed sooo... dead

Quote from DIemortalscum >>

Quote from Necropolis379 >>

so what happens if you physical form dies while you are in the astral plane? Does your soul behave like Magic Jar or do you just die?

 Well your cord would be severed sooo... dead

 As stated in the spell, when your body reaches 0 hit points, if the cord is otherwise intact your astral form is snapped back to your body and the spell ends. Death saves would then begin.

What is in the Astral plane that's its worth going to?

I think outside of anything the DM can imagine and rationalise, the intent is primarily the portals. It's kind of a travel spell, but one that can be really interesting too.

thanks

I'm currently playing a Way of the Astral Self monk, and I was wondering if using the level 18 Monk ability, or being brought along via someone else casting this would have my monk show up on the astral plane with all of their Astral self class features.

Quote from Shade11 >>

So what creatures (save for Phase Spiders) exist in the Ethereal Plane and can sever the cord? I've been looking around and can't really find anything that can.

 I haven't checked the 5th edition rules, about this, but the silver swords of the Githyanki should be capable of this.

(sorry... Was already answered) 

I am confused as to whether or not your physical body remains on the Material Plane when you enter a new plane. But the more I write this post, the clearer it is.

Your astral form can freely travel through the Astral Plane and can pass through portals there leading to any other plane. If you enter a new plane or return to the plane you were on when casting this spell, your body and possessions are transported along the silver cord, allowing you to re-enter your body as you enter the new plane.[ (I think a paragraph break should go here.)

] Your astral form is a separate incarnation. Any damage or other effects that apply to it have no effect on your physical body, nor do they persist when you return to it.

My confusion: In the spell description, we start off using the terms astral body and material body (or mortal form), then we start calling these astral form and physical body. Then I think we start using form and body totally interchangeably. Wizards of the Coast is usually highly discrete with their wording, but I only know of two entities these terms can refer to. I ... I think we can chock this up to an editing error. It's like this spell is still in its draft stages.

Also they're usually good about the word you only referring to the spellcaster. For most of the spell, you pretty clearly refers to any creature affected by this spell, until it mentions your companions in the same breath.

The resolution: If you enter a new plane, something transports along the silver cord. Contextually, I think that can only mean that the physical body moves from the plane you cast the spell on to wherever your astral form is on the new plane, "allowing" you to re-enter your body and thus end the spell. (Side note: Usually when our rules use words like allow or can, you have a choice.)

This is different from Astral Spell and Astral Projection of older editions, which have been pretty clear that a second physical body is created for you.

Astral Dreadnought 

Quote from Xix_of_Waterdeep >>

I am confused as to whether or not your physical body remains on the Material Plane when you enter a new plane. But the more I write this post, the clearer it is.

Your astral form can freely travel through the Astral Plane and can pass through portals there leading to any other plane. If you enter a new plane or return to the plane you were on when casting this spell, your body and possessions are transported along the silver cord, allowing you to re-enter your body as you enter the new plane.[ (I think a paragraph break should go here.)

] Your astral form is a separate incarnation. Any damage or other effects that apply to it have no effect on your physical body, nor do they persist when you return to it.

My confusion: In the spell description, we start off using the terms astral body and material body (or mortal form), then we start calling these astral form and physical body. Then I think we start using form and body totally interchangeably. Wizards of the Coast is usually highly discrete with their wording, but I only know of two entities these terms can refer to. I ... I think we can chock this up to an editing error. It's like this spell is still in its draft stages.

Also they're usually good about the word you only referring to the spellcaster. For most of the spell, you pretty clearly refers to any creature affected by this spell, until it mentions your companions in the same breath.

The resolution: If you enter a new plane, something transports along the silver cord. Contextually, I think that can only mean that the physical body moves from the plane you cast the spell on to wherever your astral form is on the new plane, "allowing" you to re-enter your body and thus end the spell. (Side note: Usually when our rules use words like allow or can, you have a choice.)

This is different from Astral Spell and Astral Projection of older editions, which have been pretty clear that a second physical body is created for you.

 When you go to different planes (including the Astral Plane), your body comes with you. If you cast Plane Shift, Gate, etc., you physically travel to whichever plane the spell sends you, without leaving behind a body or any of your equipment in the Material Plane. It works just like you would think without reading this spell. E.G. if a Wizard casts Gate, creating a Gate to the Nine Hells, and he and his party go through, then they are physically in the Nine Hells, along with their bodies and any equipment they bring along.

Even if you cast Plane Shift or Gate and go to the Astral Plane, your physical body is still with you, just as if you had gone to any other Plane. Your bodies come along, your equipment comes along, and you leave nothing behind, as if you had simply Teleported there, and using consumable items will consume them.

The ONLY exception is when you travel to the Astral Plane using Astral Projection. This spell works completely differently, as it leaves behind your body and equipment, creating a "projection" of them in the Astral Plane. This Projection has the "Silver Cord" mentioned, and all of the other properties mentioned in the spell. Your body and equipment remain in the Material Plane, and the spell creates "duplicates" which appear as part of your Projection. Thus, all consumable items used by your Projection are not consumed, since they aren't really there. 

Also, if you leave the Astral Plane while using this spell, your physical body will be brought to the new plane. Therefore, if you take one of the portals (called "Color Pools") out of the Astral Plane while using Astral Projection, your body is teleported to the new plane.

Hope this helped. Main source: DMG Chapter 2, with some of my own interpretation. Look in the DMG for more detail; this was my interpretation of what it said. 

Quote from BlueBurton4 >>

[ . . . ] Hope this helped. Main source: DMG Chapter 2, with some of my own interpretation. Look in the DMG for more detail; this was my interpretation of what it said. 

Quote from the Dungeon Master's Guide, Chapter 2: "Creating a Multiverse," Section 3: "Astral Plane" (p. 47)

Astral Projection

---

Traveling through the Astral Plane by means of the astral projection spell involves projecting one's consciousness there, usually in search of a gateway to an Outer Plane to visit. Since the Outer Planes are as much spiritual states of being as they are physical places, this allows a character to manifest in an Outer Plane as if he or she had physically traveled there, but as in a dream. A character's death—either in the Astral Plane or on the destination plane—causes no actual harm. Only the severing of a character's silver cord while on the Astral Plane (or the death of his or her helpless physical body on the Material Plane) can result in the character's true death. Thus, high-level characters sometimes travel to the Outer Planes by way of astral projection rather than seek out a portal or use a more direct spell.

Only a few things can sever a traveler's silver cord, the most common being a psychic wind (described below). The legendary silver swords of the githyanki also have this ability. A character who travels bodily to the Astral Plane (by means of the plane shift spell or one of the rare portals that leads directly there) has no silver cord.

Interesting. Reading this, I actually hold the other interpretation now – that travelling through the Astral Plane to one of the Outer Planes keeps your body on the Material Plane, as in earlier editions of D&D. The "spiritual states of being" and "as a dream" descriptions for the Outer Planes make me believe this spell never manipulates the home body, but the sentence "a character's death...causes no actual harm" really cinches is for me.

I am, by the way, biased towards this interpretation, because our warlock was summoned to Carcerei by his patron the Raven Queen, and we would like to pretty please return from there after and not face eternal damnation in the Tarterian Depths.

This still doesn't vibe with the PHB's spell description, where "your body and possessions are transported along the silver cord." The cord is your way back home, so any transportation along it would mean either the travel body returns to the home body, or vice versa. This holds true to the way you'd have it, BlueBurton4.

So, if someone pierced my Bag of Holding, and its contents spilled onto the Astral Plane, and I found my stuff using this spell, could I take it back to the Material Plane by grabbing my stuff in the Astral Plane, then ending the spell?

To the best of my understanding, no. Only because your Projection isn't actually your physical form, so even if you found/gathered all your lost things, you couldn't bring them back at spell's end. A Plane Shift or Gate spell that will physically bring you there would let you collect your things, but I think that would be the extent of possibility.

I'm playing a monk in my campaign. I'm far from unlocking this spell. 

This spell as a monk specifies that I can only use it on myself (as opposed to a spell caster using it and bringing people along with them). 

What in the hell would I, as a monk who cannot bring anyone else with me, and having to spend an hour to prepare this spell, use this spell for? I'm certainly not going to go to the Astral plane, or any other plane without my party a) for risk of dying in battle b) because I'm not going to have a one-off by myself while my party listens to my shenanigans in the Astral plane.

Anyone ever use this as a monk? Any seasoned DM's knowing any purpose for this as a monk? 

I too play a monk but I think this is wonderful!

The main reason being time related. Please correct me if I'm wrong, but since you are in the astral plane everything moves at the speed of thought.
Now, there are many other color pools on the astral plane. And many of those lead to the same plane but to different places on it. This holds true for the material plane too. 
So, my point being is, that you can travel to far away places in a few hours instead of weeks or months, all at the cost of 12 Ki points. 

if you are being kidnapped, or brought to another plane of existence against your will, this is your get out of jail for free card. 
If you need to travel a vast distance for information, this is your express train. 

Also I've come to understand that the astral plane is not as dangerous as many believe. Very few things can actually kill you. Most of them disorient you or drop you to 0 hit points, returning you to your body. Then you spend a short rest, rinse and repeat. 

It may not be as handy as other things but it certainly is far from useless

Disclaimer: I might be biased because I'm playing in a westmarch campaign using different planes of existence for different DMs.