It always kind of felt like Elden Ring's super hard megaboss Malenia was a transplant from Sekiro, FromSoftware's fast-paced, parry-focused ninja action game. Her aggressive swordplay could just about do laps around my sluggish Tarnished—she almost seems to demand a Wolf's precision.
Enter the Land of Reeds mod from Last孤影众 (Gūyǐng zhòng), a Chinese gamer primarily active on Bilibili. The mod seems to be a strange sort of gaiden remix of Sekiro, changing multiple areas and, most notably, porting three iconic bosses from other FromSoftware games into Sekiro, including the now iconic rot valkyrie herself, Malenia.
I'm not sure I'd recommend playing the mod itself in its current state. Its Nexus page seems to host an out-of-date version, with multiple YouTube showcases of the project instead linking to a Google Drive directory I can't personally vouch for that also lacks installation instructions, and I've seen mention of save file issues with its use. That said, I can recommend catching the mod in action from Souls YouTubers like Ongbal, a noted maestro of no-hit and challenge runs against Soulsborne bosses.
It's just kind of a thrill to see Ongbal face tank Malenia's infamous Waterfowl Dance attack with Sekiro's timed blocks—in Elden Ring, you're either dodge rolling through it like a madman, running away, or just dying straight up. Indeed, dropping the Blade of Miquella into Sekiro almost seems to make her too easy, with a little fine tuning of her stagger meter perhaps required to give her more of a fighting chance.
Ongbal also recently uploaded a video showing off Dark Souls 3's Slave Knight Gael in Sekiro via Land of Reeds, while fellow YouTuber Tigre has a no-hit run against Dark Souls 1's Artorias in the mod. The big, Guts-y swordsmen seem like less of a natural fit for Sekiro's combat than Malenia's speedy fencing, but it's still cool to see them in this completely different context.
Due to the inherent technical barrier, we'll likely always remain bereft of the true FromSoft crossover power matchup though: seeing these clowns try and take on an Armored Core. Forget augmented human C4621, callsign: "Raven," the got dang Dafeng Student Pilot, the most bullied man in Rubiconian history, could wipe the floor with every single Elden Ring demigod.
