Season 14 gives you two recipes for crafting a Mythic Unique. Enter what you're holding — the planner tracks both paths, tells you exactly what's missing, and remembers your counts between visits.
Season 14 turned Mythics from pure drops into a crafting target, and the two recipes serve different situations. The Horadric Cube's "Upgrade to Mythic" takes a Unique of 850+ item power plus 4 Pandemonium Fragments (reduced from 5 in patch 3.1.1) and returns a Mythic Unique in that same gear slot, with +30% Unique Power and the other affixes rolled at maximum. It unlocks at level 70 in Torment difficulty or higher. Read that carefully: the item you feed in is consumed as a base, not upgraded. You do not get the Mythic version of the specific Unique you put in — you get a random Mythic from that slot's pool. The targeting is by slot, not by item, so feed it the cheapest qualifying 850+ Unique you have in the slot you want, never one you would be sad to lose. Season 15 changes this, and the 3.2.0 PTR confirmed how: the Cube gains an Upgrade Direct Item to Mythic recipe that is a total 1:1 transfer — feed it a Unique with the affixes, Tempering and Masterworking you want and you get exactly that item back in Mythic form, no random roll. It costs more than the Blacksmith or Jeweler routes, it cannot be used on an Unmodifiable item, and items made this way carry a Crafted tag — you can equip only one Crafted item at a time. Mythics made by the Jeweler carry no such tag. So the equip limit removed in 3.1.1a returns in Season 15, but only for this one recipe. None of that is live yet and Season 15 has no announced date — but if you are sitting on a Unique you specifically want the Mythic version of, the current system will never give it to you and the announced one will.
The Jeweler path — 3 Resplendent Sparks and 15 runes — exists for when you don't own a strong base in the slot you're chasing. It's the more expensive route in raw material rarity, but it doesn't ask you to first farm the 850+ Unique.
Fragments come from the seasonal loop: finish the "A Gospel of Despair" questline, then run Pandemonium Ruptures into the Realmwalker and Deathtoll Chambers — Fragments accumulate through that circuit alongside Superior Lair Keys. Once crafted, your Mythic isn't finished: Season 14 also lets Mythics be Enchanted and Tempered, which is where the last chunk of power lives. For the wider crafting system, see the Horadric Cube guide.
No — it upgrades the specific 850+ Unique you feed it, same slot. That's the whole point of the path: bring the item you actually want as a Mythic.
Yes, locally in your browser — come back mid-season and your material counts are where you left them. Nothing is sent anywhere.
The recipe cost is 15 runes alongside the 3 Sparks. Rune values differ wildly on the market, so feed it your cheap ones — the recipe doesn't reward premium runes.
That depends on your build — check your class's expanded card on the tier list for its S14 build guides, which list the Mythic priorities per build.