You can push Diablo 4's Charm system even further with this simple trick
A clever interaction between two Mythic Seals lets you equip up to three Unique Charms and a full Set bonus.
The 
Hellfire Torch is probably Diablo 4 Season 15's most powerful Mythic Charm, but there is a small interaction that makes it even more useful. By combining the Hellfire Torch with two Mythic Seals in a specific order, you can equip multiple Unique Charms and still keep your full five-piece Charm Set bonus. Blizzard knows about the interaction and has confirmed that it goes beyond what was originally intended, but they decided to keep it for Season 15. So since we are officially allowed to use it, we figured we would let you in on how exactly it works.
The Trick Uses Two Mythic Seals
The important pieces you need are the 
Seal of the Diamond Mind, 
Seal of the Golden Epiphany, and the Hellfire Torch. So yes, you might need to go farm a bit first. The Torch can only drop from the Uber Prime Evil fight, and for the Mythic Seals, Baal is a pretty good farming place.
Now about the specific interaction between all these items. Golden Epiphany allows you to equip up to three Unique Charms. Diamond Mind, meanwhile, reduces the number of Charms required to complete a Set bonus. And you need a Hellfire Torch that rolled with the Set Item bonus that you need.
The trick is that the game does not completely forget your Unique Charm setup when you switch between the two Seals.
Equip the
Seal of the Golden Epiphany firstPlace your
Hellfire Torch and two other Unique Charms into your Charm slotsSwap Golden Epiphany for
Seal of the Diamond MindDo not touch your Unique charms!
Fill the remaining slots with your Set Charms
It is important that you DO NOT remove or replace any of the Unique Charms. Because otherwise you have to slot back Golden Epiphany to be able to activate 3 Charms again. If everything is done in the right order, the game keeps all three Unique Charms active, and you still keep your full five-piece Set bonus at the same time.
When you swap to Diamond Mind, if one of your Unique Charms deactivates, just redo it and make sure you place Hellfire in the middle down. For some reason, this seems to work better. So you get something like this:
That trick can push the Charm system considerably further than a normal setup would allow. You can squeeze way more power out of your build with this! We would recommend not competing for higher Tower leaderboard positions with this, as it is ultimately not the fairest thing to do, because even if Blizzard allowed the interaction to stay for this season, it is not something that was intended. But if you want to see how far you can push your build with this, it is a great way to have some fun!
The problem is, of course, getting both Mythic Seals in the first place. Golden Epiphany and Diamond Mind are already extremely valuable items on their own, so this is not exactly a trick every character can immediately take advantage of. Still, if you happen to have both, it is worth knowing that you can use both.