Are these Bracers of Blinding Strike balanced?

Question

I am attempting to build a magic item for a player request, and I thought I would check with the community for balance issues. I decided to use the flavor of the original books, rather than copy older rules.

In the books Drizzt found that the bracers were superfluous when speeding up his hands, and so he wore them on his feet instead. With that in mind I decided that granting some of the effects of a haste spell, split between hands and feet might be the way to go.

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This pair of elegant bracers is crafted from green leather with mithral tracery that looks like vines. Tiny, leaf-shaped plates of silver branch off the mithral to create a glimmer­ing pattern.

Bracers Of Blinding Strike

• Attunement required
• Adventuring gear (wondrous item, arms or ankles)
• Category: Items
• Item Rarity: Legendary
• Modifiers: Initiative +2

When worn, the bracers offer the user a +2 initiative bonus.
If worn on the wrists, the bracers grant the users the Extra attack as the 5th level fighter ability. Multiple uses of this feature from any source do not add additional attacks. If worn on the ankles, the bracers grant the wearer +10' to base movement speed.

Answer 1

Extra Attack is potentially a very strong ability. As one example, if a rogue wears this, they could make their shortbow attack a second time each turn, adding 1d6+DEX damage every turn and also giving an increased chance to trigger sneak attack.

You've told us that the player requesting this has a wizard character, which surprises me. Are they planning to use this with shadow blade? This item could double their shadow blade damage, which is pretty good -- except that wading into melee combat is sort of a questionable plan for a wizard, and shadow blade damage isn't high enough to make up for that as far as I can tell.

I can't think of any other use for this item, for a wizard. I checked if it was possible to polymorph into a monster that could wear this item usefully, but most monsters have a multiattack power already. I checked if this could be used with the weapon attacks from tenser's transformation, but that spell grants Extra Attack already, so this item wouldn't help.

I think this item is too good for general use because it buffs the rogue too much, but I don't see any obvious problems with giving it to a wizard.

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You've asked if this item is broken in general, but I think a better question is if giving your wizard a slightly-overpowered item will unbalance your game. Is your wizard player a skilled optimizer? Is the wizard currently dealing more damage than other characters, or less damage?

(Have you tried directly asking your wizard player why he wants this, and what he plans to use it with? This is a good practice because he might have some rules misunderstanding, like he might think it gives him an extra scorching ray attack or something, and you'd want to clear up that misunderstanding before he commits to the item.)

I'd recommend you make your decision based on how well-optimized your wizard character is, compared to the rest of the group.