Punishing permadeath platformer Dead Cells (our pick for Best Roguelike back in 2018) just got a new update that adds skins, weapons, and skills from other indie games that have cores of similar hardness. To unlock the outfits you'll need to complete challenges found via a book of clues in the opening area, and the rest can be found in new lore rooms scattered throughout the levels.
The additions include the nail from Hollow Knight, which can be used in a downward-attack move I'm sure won't be infuriatingly difficult to pull off. Though the Knight himself isn't playable in Dead Cells, Juan from Guacamelee is and thanks to "Pollo Power" he can transform into a chicken that lays eggs, which then explode. That's a tough act to follow, but the Penitent One from Blasphemous does add a Face Flask you use by smashing it onto your own forehead.
There's also a gun from Hyper Light Drifter (which marks targets, and can turn into a sword that causes bonus damage to said marked targets while also regenerating the gun's ammo), a bone club wielded by Skul: The Hero Slayer (who is a skeleton, it's like if you or I went into battle wielding a severed leg), and the mutton-chopped protagonist of Curse of the Dead Gods brings a machete/pistol combo. That last one returns the favor from the time a bunch of Dead Cells gear appeared in Curse of the Dead Gods.
I find these kinds of crossover events between indie developers charming. When, say, Terraria and Don't Starve have a crossover it's presumably because of mutual respect between the creators rather than a big wodge of money being handed over to get another corporate entity jammed into Fortnite.