If you've just found the Imbued Sword Key in : , your first thought was probably "Oh hells, [[link]] not this again" followed shortly after by wondering where to find the Belfry you can use the key. For those familiar with the base game, Elden Ring has a location called the Four Belfries in Liurnia that lets you use Imbued Sword Keys to open portals.
These send you to self-contained mystery locations all over the map, and are a fun way to get a sneak peak of what the game still has in store. Surprisingly, Shadow of the Erdtree also has a teleportation Belfry, though there's only one portal in it. Here I'll run through where you can find the portal, plus the location of the Imbued Sword Key if you don't have it yet and want a mystery tour to a random place.
Where to find the Imbued Sword Key in Shadow of the Erdtree
You can find the Imbued Sword Key in Castle Ensis, not that long before its final boss. If you haven't visited Ensis yet, you just need to cross the Ellac Greatbridge in the north of Gravesite Plain, and then continue onto the castle ahead. Once inside, you'll have to beat Moonrithyll, Carian Knight, and then head through the chapel and continue along the route until you hit a metal gate.
Just next to the gate is some scaffolding you can climb by walking up a pike leaning against it, and then you can hop over the gate to continue along the battlements until you find a Black Knight barring your path. Now, look directly behind you to see a small room with a chest containing the Imbued Sword Key.
Where to find the Belfry in Shadow of the Erdtree
You can find the teleportation Belfry in Scadu Altus, where you can use the key. You'll arrive here after you defeat the final boss of Castle Ensis and will emerge into the region at the Highroad Cross site of grace. From the Miquella Cross there, look directly to the west and you'll see the top of a small stone structure rising up beyond the edge of the hill.
Jog on over and you'll spy a little island with a river running either side in which a Carian spirit troll is patrolling. Congrats! You've found the teleportation Belfry. [[link]] Head on up to the bottom of the stone structure and insert the Imbued Sword Key into the imp statue to open the portal—there's only one this time alas.
I won't spoil where you're going because I'm not [[link]] a monster, but one thing I will say is make sure to check the far back of the area from where you arrive to find a fun talisman.