All unofficial universes by Google.
Probably the best known.
"John Carpenter's Apocalypse Trilogy"
The Thing (1982), Prince of Darkness (1987), and In the Mouth of Madness (1994)
Next one comes from reddit:
"Destructive Power of the Ancient Occult"
The Autopsy of Jane Doe (2016), The Blair Witch Project (1999), and Hereditary (2018)
I always thought they were similar long before the internet decided they are part of an unofficial universe.
"Sci-Fi Hell"
Event Horizon (1997) and Hellraiser (1987)
I wasn't aware of the next one.
"The Ultimate Halloween Anthology"
Trick 'r Treat (2007) and The Town That Dreaded Sundown (2014)
and
"The Industrial Sadism Matrix"
Cube (1997) and Saw (2004)
If anything, they should make fir a fun Halloween watch. Do you know of any unofficial horror movie universes?
Bride of Chucky, Chucky's evidence locker includes Freddy's glove, Jason's mask, Michael Myers' mask, and Leatherface's chainsaw.
Also, The Rise of Leslie Vernon treats Michael Myers, Jason Voorhees, and Freddy Krueger as real historical killers.
On top of that, Freddy's glove can be seen in the Evil Dead Cabin and the necronomicon and Kanderian dagger from Evil Dead are seen in Jason goes to hell, which also shows Freddy at the end.
So ...
Friday the 13th
Nightmare on Elm Street
Child's Play
Texas Chainsaw Massacre
Halloween
Evil Dead
Rise of Leslie Vernon
...
all part of the same Universe.
... and if you want to count comicbook crossovers, we can add...
Vampirella
Evil Ernie
Hack/Slash
Hatchet
Re-animator
Nightbreed
The Crow