Found behind the Wyndham Ruins on the Altus Plateau, these catacombs are lightning-themed, both in the loot you find and the enemies you meet along the way. You also get access to a bell bearing that’ll let you purchase infinite low-tier Glovewort.
From the site of grace, continue down the stairs and you’ll see the boss doors. Don’t forget to activate the summoning pool if you want to co-op here later (this boss is pretty easy). Look on the ground in the eastern hallway — there’s a square trap trigger there, although if you let it hit you it’s not too far to go back and heal at the site of grace. Past the trap is an elevator, so hop on and start walking into the western wall — near the bottom, you’ll walk off onto a ledge that has a Grave Glovewort . Then drop down, send the elevator back up (and jump off), then continue east down the stairs.
In the next room, there are miraculously no enemies hiding on either side of the door. There is a blade trap in the middle of the room, and two gargoyle imps that’ll throw lightning pots at you. If you’re a melee character, wait for the blade to drop before running past it and engaging the imps. Grab the Grave Glovewort , then head east into the hallway. Turn the corner to continue south, and you’ll see an imp on the wall in the next room. There’s another to the right of the entrance, so take that one out first before dealing with the imp on the wall. Pick the Grave Glovewort  in this room and then go down the stairs that lead further south.
When you get to the bottom of the stairs and enter the watery area, don’t go straight — instead, go right or left first to avoid one of the slime monsters glomming onto your head when it drops from the ceiling. Don’t attack any of the slime blobs here — they’re going to help us in a little bit. Note the trap on the ground in the middle of the area, and continue south.
Pick the Ghost Glovewort  just past the trap, then turn right to head west, where you’ll find a lightning knight. Get his attention and then run through the zombies and back to the trap. Wait for the knight to be close to the trap, then set it off and run past the slime monsters. Once the knight is close to them, try to run around him — sometimes, he’ll target the slimes instead of you, and you can shoot/stab him in the back. You can also just use the trap over and over if the slimes die and the knight is still alive. Once the knight perishes, head back to where he started, pluck the nearby Grave Glovewort  then climb the ladder the knight was guarding.
At the top of the ladder, you’ll see a double stonesword imp statue and its accompanying fog gate. Approach carefully, as there is another trap on the floor, and an imp on the ceiling just in front of the fog gate. Inside is the Lightning Scorpion Charm — useful if you use lightning weapons or spells. Head south from the imp statue into the next hallway, but stop at the end of the hallway and read on. The next room has a trap floor that will raise into a ceiling of spikes, and we’ve got to clear a gargoyle from above us before we cross it.
Step out onto the floor and turn around — the floor will rise and you’ll find yourself face-to-stone face with a gargoyle imp. Move off of the floor and into the alcove with the imp, then kill said imp (it should still be pretending to be a gargoyle). Look south and you’ll see a hallway with another imp — once the floor resets, drop down onto it and sprint towards the hallway. Take out the imp and then continue down the hallway. In the next room, another imp hides on the right, and a lightning knight guards the lever. Kill the gargoyle, and then you can either kill the knight, or lead him away from the lever and then roll past him to activate it (though you’ll then have to immediately roll/run away) — this lever will open the boss doors.
Either way, once the lever is pulled, go back north through the hallway and drop down, then sprint across to the alcove. Wait for the floor to reset, then drop down and immediately run/roll north (to enter the hallway underneath the alcove). Then, you can drop down under the rising floor to the ground below. (If you miss your chance, just step out onto the floor and then roll off back into the hallway, and you’ll be able to go under the floor).
Head east to leave the floor trap room and enter another watery room full of zombies, with another trap on the room’s centerline. Walk slowly through the water until you find the trap, and then clear the zombies (or use the trap to do so). You’ll see some items on the other side of the room — two crabs will come out of the ground when you get near the items. You can lead them back to the trap and use it to damage them, or you can fight them old school (or you can grab the items, avoid the crabs, pull the lever, and go through the now-open metal gate from earlier).
The three corpses hold x2 Magic Grease, a Golden Rune , and the Ancient Dragon Apostle’s Cookbook , so make sure you get all three before pulling the lever (which is to the right of the gate) and heading through the metal gate. On the other side of the gate, continue north, up the stairs, and make your way back to the beginning of the dungeon, where you can head through the boss doors and face the lightning version of the Erdtree Burial Watchdog.