Edit: This deck made it to Champion.
Despite the nerf to Allfather's Horn this deck is back and better than before. Rings of Immortality has given it the tools it needs to stall until its combo is in place with surprising consistency. The winning plan however is still the same: Play Radiant Vessel, keep it on board until your next turn (often with the help of Soma Oasis), then have it consume your orange gems, cast Horn, have it consume your orange gems again, then slam it into your opponent's face twice (often with the help of Seahaven and/or Impel) dealing 6 + 4 times your number of orange gems in damage all at once. Your opponent typically won't see this coming and will be completely blindsided by a sudden loss in a game they thought they were winning.
If you want to know how to play this deck in more detail, read what I wrote here https://www.mythgardhub.com/deck?id=2080 about the pre-Rings of Immortality version. I'll mostly focus on the changes post-RoI in this deck's description, but these are the essentials: burn your 1-2 drops or non-combo enchantments when possible, go for the combo as soon as you feel like its fairly safe from removal, and play defensively in the meantime in order to last until you get all the pieces. If your only chance of winning is to combo immediately, go for it no matter how unlikely it is to work.
Horn now costs 7. You may think this is a nerf, but the deck only somewhat cares. You want to attempt the combo as soon as you feel like you have a fairly safe shot at it, but that usually takes 7 or more turns anyway. If you have Oasis and Seahaven in hand and your opponent isn't telegraphing an obvious way of shutting Vessel down, you're probably safe. If your opponent is yellow, also consider waiting for Hammam Retreat. If you don't think your opponent has damage/destruction based removal, consider NOT waiting for Oasis. If you have multiple Vessels in hand, you can afford to play them in much more risky scenarios. Plus, there's a whole bunch of ways you can setup the board to increase your odds, even without the specific supporting enchantments.
The Peri nerf didn't affect the deck much either. She's actually slightly better for searching up specific combo pieces, and has more bulk to help survive whatever your opponent is doing. You can still often get her second draw by putting an enchantment back on top of your deck, but losing that guarantee does sting. Peri is a different card now, but she's still one the deck wants to run.
The 3 Einherjar Thanes of the old version have been replaced with 2 Wry Tricksters. Since the only reason you run early game minions is to do a little incidental damage and prevent your opponent from snowballing the board out of your control early, Trickster is just better in every way. It can contest everything without Rush that your opponent could play on turns 1-2 super well.
Solomon's Gale is gone and 2 Jaza'eri Arquebus take its place, plus the slot left over from losing a 2-drop. Arquebus does a much better job defending the board, benefits twice from all your enchantments, and costs 1 less. Gale's ability to occasionally go out of control is too inconsistent to be worth it in comparison to what is basically Mythgard's Piloted Shredder.
Speedway is gone and replaced by Higher Ground. Speedway's two blue gems made it impossible to follow up a Recruit with, and it was too inconsistent as a tool for letting Vessel connect, since any blocker with more than 2 strength still killed it before it could land the second hit. In contrast, the Divination that Higher Ground occasionally grants is amazing. If your opponent doesn't contest one of your proactive 1-2 drops, placing this beneath it is a good idea. It's still one of your main burn targets, but now even that is rewarded by the card's Forge ability.
Three of the Bazaars are gone in place of Hammam Retreat. One of the cards that could ruin your day before was Misanthropia, and now it isn't as big of a problem. Retreat won't be relevant in all matchups, but it's still better than the rarely played Bazaar. You can also stack the protective effect of both it and Oasis by playing Vessel on Oasis then Impelling it onto Retreat.
In my previous write-up I recommended dropping Soma Oasis ahead of time. This is because the ideal scenario for the old deck was turn 6 Horn, and you couldn't play both Oasis and Vessel on turn 5. Now I simply recommend dropping both Oasis and Vessel the turn before you hope to pull off the combo.
As always, there are a million little things you'll learn to do playing your deck. That's Mythgard. This deck in particular is an odds game, some matches you are destined to lose and others you are destined to win regardless of your opponent's skill. I suspect I wouldn't have fun losing against this deck in a million years, but being the one in the driver's seat is an absolute ball. I might be making a mistake by putting it out here. Good luck.