The lucky coin questline that has culminated the base game and both expansions has finally been solved! By taking the lucky coin, luckier coin, and luckiest coin - you can acquire Ring of Misfortune, Ring of the Jynx, and Amulet of Dark Omen. With these items equipped, you gain access to a hidden boss fight - against all the Horadrim we've lost along the way.
The questline is quite involved and requires friends along the way to unlock the Uber Horadrim fight! Join my Discord to find a party to unlock it. I'll also be getting people in on my Twitch Stream
Discovery Credit: Not Finding a Cow Level Community
To unlock this , you need to complete all 3 questlines. The first of which is the Lucky Coin.
Obtaining the Lucky Coin
First off, you need to obtain a greed shrine. With it enabled, you have to loot the corpse of an enemy that was killed by an NPC event for a chance to loot the Lucky Coin off their corpse. These events tend to occur most during Helltides in Skartara. Having 
Ahavarion, Spear of Lycander makes it easier to obtain the Greed shrine effect.
Getting the Ring of Misfortune
With that coin in hand, you need to head to a Wishing Well in Diablo 4 - in Scosglen, east of Cerrigar. Here, you drop the coin in the well for a wellwisher achievement. Now that’s not the end of it. If you drop the coin alongside 4 others - for a total of 5 - people playing different classes - a unique ring titled the Ring of Misfortune will drop as a result, concluding this questline
Obtaining the Luckier Coin
The second part of the questline is in Vessel of Hatred - the ‘luckier coin’ hunt. This questline is in Nahantu - and requires you to obtain a Blast Wave shrine and break highlightable containers with it (as in you can hover over them). A tip here - group up with a party member and have them stand by destructible objects. Then go and find a blast wave shrine. Once you do, teleport to them and break it that way. Upon doing so, a rat will eventually spawn that’s unkillable. At this point, all you can do is follow it - and after 7 minutes and 9 seconds exactly - the rat will explode, dropping the luckier coin.
Getting the Ring of the Jinx
Upon obtaining this coin, you need to go to the town of Samuk - where you can now interact with a moss-covered grave. This drops another unique ring - this one called the Ring of the Jinx - which will break objects around you
Obtaining the Luckiest Coin
The luckiest coin requires you to be in a group of 4 and simultaneously have each party member kill one of four different golems located in Skartara:
* Raging Fissure
* Flowing Rage
* Droplet
* Snagging Vein
The map above indicates their exact locations, each representing a different element. Upon successfully killing them all at the same time, each member will obtain the Luckiest Coin
Getting the Amulet of Dark Omen
After receiving the Luckiest Coin, head to Temis and go further down the path past The Aegoye dungeon, which is near the bridge to the island of Lycander. Here you'll find a bed, and with the luckiest coin in hand - you can interact with the pillow here. Now - you must wait. After exactly 24 hours (you only have a 2 minute grace window), you can interact with the pillow again and obtain the Amulet of Dark Omen.
Access the Uber Horadrim Fight
To access the Uber Horadrim fight - travel to Lorath's Cabin, which is inbetween the Nevesk and Kyovashad waypoints - refer to the mini map above. Inside the cabin, wear all 3 jewelry from the quest above: Ring of Misfortune, Ring of the Jynx, Amulet of Dark Omens. Upon doing so, a horadric portal will open up. Inside, you'll find a foyer, and in the middle you'll be faced with the echoes of Donan, Lorath, and Neyrelle. Note that once the portal is open, you can permanently access this fight, however, if you or a party number does not have the jewelry equipped, they will not get any loot
The Horadric Coin Recipe
Upon defeating them, you'll be rewarded a boss trophy titled the Horadric Coin. This will allow you to purchase these items once more on the next season - even allowing you to get Ancestral or Mythics versions, which can be a nice boost early on in the season. Transmute the coin in the Horadric Cube with 3 junk items obtained from fishing - 2 engagement rings and a Lost Amulet. When you transmute it, you'll obtain the Sack of Ill Trinkets - which contains the 2 rings and amulet you need to open the portal once more
Once you do this, you can buy the Sack of Ill Trinkets anytime at Squirt the Peddler, located by a fountain in Temis west of the Waypoint
This quest is a bit involved and requires a good amount of luck to get the shrines aligned with the proper events required to score the coins. I guess that makes those coins aptly named!
My community is helping people farm the coins and granting direct access to both the Uber Horadrim fight here as well as granting Cow Level items on both Discord and live on Twitch - go ahead and join and I'll see you there!
-Loathe