This article will contain all manner of spoilers for Elden Ring; there is no way around that. What I will promise is that I will try and divvy them out in order of impact and keep them as vague as possible. As soon as you know the information you need, you can bounce out and hopefully maintain some mystery.
How to get Miquella’s Great Rune
To get Miquella’s Great Rune, you will need to defeat the Scadutree Avatar that can be found after the Tree-Worship Passage Site of Grace. Getting to the Tree-Worship Passage is quite the trek. First, you need to make your way to Bonny Village, where you can find the O Mother gesture.
From there, you need to go all the way up the Church District Highroad to the back of the Shadow Keep and in the Church District Entrance. There will be a flooded area to traverse, and you will need to lower the water level. Assuming that you have both of those done, I can lead you the rest of the way from here. If not, see the above-linked guides for step-by-step help.
Once you have lowered the water level, drop back down the ladder that led you to the lever and go through the door directly beside it. There are no stairs on the other side, so carefully drop down to the platform below and then to the ground.
There will be a short set of steps on your right side as you come out, so take them up to the next Site of Grace, called Sunken Chapel. From there, venture forward and out the other side of the building, and you will see a central circle area with a tree inside it on the left side.
You need to go through here but be very cautious. A giant beast will appear when you try to get through, and you will need to deal with it or run if you can. When the fight is done, head through the door up the steps on the far side. You will arrive in a large room with a caster enemy at the end. Some of the little exploders will appear, so be careful.
From the end of that room, before you go into the room with the statue of Marika, you can take a right and then go all the way to the end, where you will find some locked double doors. Open them and take the elevator down, and you will come to the Tree-Worship Passage Site of Grace. Continue down the tunnel, across the bridge, and into the room with the huge statue. Grab the Iris of Grace in there and then go out the other side and activate the new Site of Grace. Coming up is a very tough boss fight against the Scadutree Avatar at the Scadutree Base. When you finally kill that huge beast, you’ll get Miquella’s Great Rune.
This fight is a tough one, but fire damage is definitely your friend, so break out any magic or grease that will give you some fire attacks and just go ham on this errant tree.
What does Miquella’s Great Rune do? (big spoiler ahead)
Retains naught but the power to resist charms.
This is a useful item for the last boss fight in the game. During that fight, your enemy can reach out and grab you. It doesn’t do any damage, but it will charm you. If you do not use Miquella’s Great Rune to dispel that charm, you will automatically die the second time it happens.
How to use Miquella’s Great Rune
This is NOT a Great Rune in the same way other Great Runes are. You do not need to visit a tower and attune to it. Go into your Equipment, and you can equip it as an item, the same as Torrent or some Ashes. Just add it to your inventory items, and you can select it mid-fight and use it. It doesn’t take too long to get through the animation, but when you need it, you will need to buy yourself a bit of space.
In-game lore
A Great Rune relinquished by Miquella. Broken and bereft of its bounty, it retains naught but the power to resist charms. Miquella set off for the tower enshrouded by shadow, abandoning everything—his golden flesh, his blinding strength, even his fate. All in an effort to bury the original sin. To embrace the whole of it, and be reborn as a new god.
