ehh looks like priest vestments. not sure if i like it

The image displayed is just an example of the Good alignment version. They don't all necessarily look like this. It's D&D you can have it look like whatever you want.

Well I don't imagine they would have various different images all here. This is just an example of what a good-aligned robe might look like.

This is OP for a dex based character.

Yes.  However, this can only be attuned by a wizard, sorcerer, or warlock.  None of those classes tend to be dexterity based.

Only if you min/max the character. You can quite easily roll high stats and in turn have a high dexterity. Or some may just want a high dexterity and sacrifice a higher score in another ability, depending on the way they choose to play their character.

Multiclassing anyone?

Fair point.  Granted, I'd have decently high Dex for a wizard anyway, since there's no reason not to.

BLADESINGER

I had this item discovered by my party wizard at level 14. She was Lawful Neutral, the Robes were black. We had about three sessions, where she was struggling internally whether to fall towards evil for the sake of gaining power (Which the party were in dire need of), or staying true to herself. In the end, she chose instead to dedicate herself to being a GOOD aligned character, disgusted that her moral grayness had almost lead her towards actual acts of villainy. What I'd planned as a 5/10 minute temptation ploy turned into 3 sessions of amazing and satisfying character growth, so of COURSE I had to reward her, by having the cloak actually converted to Good alignment, having been moved by her. Maybe it wasn't RAW,  but it was immensly satisfying for her and the party, so I regret it not one bit :)

Quote from Love_and_Stuff >>

I had this item discovered by my party wizard at level 14. She was Lawful Neutral, the Robes were black. We had about three sessions, where she was struggling internally whether to fall towards evil for the sake of gaining power (Which the party were in dire need of), or staying true to herself. In the end, she chose instead to dedicate herself to being a GOOD aligned character, disgusted that her moral grayness had almost lead her towards actual acts of villainy. What I'd planned as a 5/10 minute temptation ploy turned into 3 sessions of amazing and satisfying character growth, so of COURSE I had to reward her, by having the cloak actually converted to Good alignment, having been moved by her. Maybe it wasn't RAW,  but it was immensly satisfying for her and the party, so I regret it not one bit :)

Wow, what a great anecdote. Great DMing on your part (assuming you were the DM). IMO, good roleplay should (almost) always be rewarded like this. 

So, for my 20th level party, the BBEG of our campaign is Nether, God of Necromancy. He’s a Lich with:

two 9th level spell slots

A robe of the Archmagi

A staff of the Magi

they might die...😰

Rip lel. 

So could a Neutral Good character where wear a white or gray robe? Or neutral evil gray or black? Or is it blank good, blank evil or blank neutral?

Quote from Actorclown >>

So could a Neutral Good character where wear a white or gray robe? Or neutral evil gray or black? Or is it blank good, blank evil or blank neutral?

 I think that if you have a NL character it can have white or gray

Lawful Neutral and Chaotic Good characters must use white
Lawful True and Chaotic Neutral characters must use grey
Lawful, Neutral and Chaotic Evil characters must use black

It's the base alignment that it's dependant on. Not the secondary alignment classing

Add an army of Death Kinights and Liches riding Dracoliches and they will die.

Will this work for Rouge: Arcane Trickster?

(requires attunement by a sorcerer, warlock, or wizard)

Insane for a Hexblade or Bladesinger

How would y'all define "magical effects"? Dragon's Breath? A hag's Frightful Gaze? I'm playing an abjuration wizard so I already have advantage against spells, I'm confused what else could be covered.