No sure if this effect would be better off on a legendary enchantment.

All nonland permanents are legendary? Sure, that's Leyline of Singularity, a fun combo piece and token hoser.

All spells are legendary? As in Legendary Sorcery as in Jaya's Immolating Inferno? That's... well, at first it feels like a big thing, but then in conjunction with the first bit, it just means "you need to have a creature to cast an instant or sorcery". Which is probably okay actually, especially on a white rule-setting card. Fun!

If anything this is overcosted though. I'd expect this to be 5 mana at rare, or probably 4 mana at mythic. (And I hate that a card's cost is so correlated with its rarity, but it's the sad truth.)

You're right it is overcosted or not necessarily a mythic given everything else you've said. But maybe "all spells are legendary" doesn't properly do what I intend. Something like "players can't play spells with the same name" + something to ensure the exile of duplicates, would be more of a real lock.

I think this is better as something riffing off of Eye of Singularity, but with spells added.

Oh, it feels mythic. "Everything is Legendary. Yup, even spells". Impactful as heck.

But if your aim is to enforce the original legend rule on spells in flight; that doesn't really help anything? Because how often are multiple copies of the same spell in play at the same time? I guess it hoses storm nicely; but that's a bit niche. (Good in the right set though.)

I like this as it is. "You're all legends. And sneakily, that means you need a creature in play to cast anything. And, oh look - I have this creature in play. Now to do something about yours..."

To actually get "You can't play a spell twice" you need to check graveyards. Which would be doable? But less "Wow, few words, big impact" mythic.

True that, it is still pretty impactful.

I think the temptation here was for me to go for something like another design of mine from a few years back that was something like this:

"When X enters the battlefield, exile all cards from all graveyards. Whenever a player plays an instant or sorcery, exile it as it resolves. Players can't play instants and sorceries with the same name as a spell exiled by X."

That truly makes instants and sorceries "legendary" in a "there can truly only be one" sense and is more of a "hard lock". It is definitely more wordy though and feels more appropriate on an enchantment.

Huh, I thought there was a wacky old enchantment that said "Nobody can cast spells with the same name as a card in a graveyard". I think I must have been thinking of Cornered Market, but that only affects permanents, not instants or sorceries. (Well, unless you make a Splinter token with Splintering Wind.)

Yea I have had cards like that in the back of my mind that I might want to make a variation of, possibly for instants/sorceries.

Anyone else bothered by those names?

What's the issue with the names? Or you mean the dashes?