Dope, using this

Perfect for my dnd character 

The astral plane part is very interesting...but your character wouldn't know what was going on in the material plane once they leave it, right? It's not like casting Blink where you can see where you came from?

That's the bit that worries me. I can't tell if this is meant to be used in combat or out.

It's all fun and games until the Wizard encounters an Astral Dreadnought...

I would say the character couldn’t see onto their home plane while in the Astral Plane. I feel like it’s meant to be sort of like Astral Projection: an exploration tool, rather than a combat tool. Although you could use it to hide away and protect your concentration if you were holding up a spell like Wall of Force or Banishment.

A bit new to D&D and loving it. I am wondering if it's cool for my Rogue character to use a Robe of Stars he found once he gets to Level 10 (I think) and gets "Use Magical Item" as a feat. I can't find any specific guidelines on that and any help would be appreciated!

Quote from Eamonkin >>

A bit new to D&D and loving it. I am wondering if it's cool for my Rogue character to use a Robe of Stars he found once he gets to Level 10 (I think) and gets "Use Magical Item" as a feat. I can't find any specific guidelines on that and any help would be appreciated!

Items will have specific attunement requirements if applicable. For example Staff of the Magi says it can only be attuned to by a Sorcerer, Warlock, or Wizard. Since this robe doesn't have any form of attunement restriction there should be no issue with your Rogue using this item.

Cast Polymorph on target, turn it into a fish.

Pick up fish, use action to enter astral plane.

Toss the fish away from you, use action to return to where you started. 

My bard is absolutely loving this as a form of damage while maintaining her concentration spells. Why I was given it at level 5 I don't know, but I won't question the DM (or my party, who didn't really bat an eye over wanting it). And, my character is just itching for some down time to go explore the Astral Plane. The 1 action to get out is helpful too. 10/10, if you have a curious character who also needs a smidge of extra damaging spells amidst numerous concentration supports, this item is brilliant.

That being said, it does seem to be bugged in D&D Beyond's system.

The wording says, "while wearing this robe, you can use an action to pull off one of the stars and use it to cast magic missile as a 5th-level spell" (bolded emphasis mine). However, when I cast it from my spell list using the item's charges, it rolls 1d4+1 for damage, and marks off a charge from the robe. Casting Magic Missile at 5th level results in 7 force darts (7d4+7 damage total, or split between enemies).

Magic Missile is a spell you can split between as many enemies as there are darts if you please, so maybe that's why it does this, but if you roll for each individual dart using the spell slot it gives you, it will consume all 6 charges of the robe, when you only use one to cast the spell at 5th level. My DM tried fiddling with the item to see if he could customize it, but no dice (pun intended). Not sure if it's fixable, just a heads up to any who plan on using it. 

the charges and the dice roll are two separate entities and not connected for a reason. Each dart can hit ANY target in range, so each dart is always rolled separately for damage purposes.

while most people just anime blast all the darts of a single casting into a single target, because of how the spell is worded, and mechanically used, it is too much trouble to make a weird extra Magic Missile dice set up for this one spell.

The solution is to just click the dice rolling option 7 times and track the total. A very slight annoyance, and really not that inconvenient.

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Unrelated to the above, WHERE in the Astral Plane do you go? Can you select? is it always random? Do you just sort of go... nowhere? (I know, up to the DM, but really... I am scurred to use this, but also DESPERATELY want to do so!)

Magic Missile automatically hits, unless the enemy uses a reaction like the spell shield, which negates it. Shouldn’t have to roll anything. Just FYI.

Correct. The damage still needs rolled though, and the other commenter suggested using the manual rolling option just to save the inconvenience I mentioned in my first comment.

Ah, I read your comment wrong. I thought you said rolling for attack as in to hit. My bad lol

this is stupidly powerful. Guess I'll check my local dnd shop to see if I can buy this.

The reason why the sheet would only roll the 1d4+1 is because that's the RAW. You roll that once, and each dart does that. If you rolled 1d4+1 and resulted in 3, then all darts do 3 damage each. At 5th level that is 7 darts, so if you got 3 that's 21 damage to split in thirds between targets. 

Can concentration spells be maintained from another plane?  For example, could I cast Sickening Radiance and then go Astral the next turn while maintaining the effect of the spell?

From pg196 of the PHB: Damage Rolls: "If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them." 

Technically, Magic Missile rolls 1d4+1 and applies that roll to all the darts which hit at the same time. As per damage roll rules in the PHB, since its a spell that hits multiple targets at once you only roll damage once. That's also why a similar spell like Scorching Ray asks you to roll a separate attack for each target. The spell is working in Beyond as intended as far as RAW goes. When you cast a spell, you do what the spell description says and no more, which is exactly what Beyond is doing. Here's a link to Jeremy Crawford's answer to this question : https://www.sageadvice.eu/magic-missile-do-you-roll-the-same-d4-for-all-darts/ . He also covered it on an old episode of Sage Advice (part of the Dragon Talk podcast) and went into more detail about why spells are designed this way and what other commonly overlooked rules people miss.

All that being said, at a real table with real dice, what kind of monster isn't going to let you roll all those d4s?

you're  supposed to just use that damage on all seven missiles separately.

I have no idea why it does this.

I think, RAW, that is how you are supposed to calculate Magic Missile damage…roll 1d4+1 and multiply it times how many darts are being cast. Todd Kenreck did a video on why this is the case based on the wording of the spell(I believe Jeremy Crawford has weighed in as well) even though they both admitted that at the majority of tables, the damage dice are rolled separately for each dart.