Just wanted to make a modpack for October 2025. I highly recommend using the shaders included in the modpack (unless you computer can't handle it), since it greatly enhances the spooky vibe. The only config change I made was in Serene Seasons, where it starts in late autumn and the subseasons last 999 days (and time doesn't pass when no players are online). All credit goes to the creators of the mods used.
Main Mods:
Autumnity - Adds new biomes and other autumnal features
Born in Chaos - Adds spooky mobs and structures. New armor and weapons.
Haunted Harvest - Adds autumnal/Halloween activities and features
Intense Horror - Adds a "night of fright" where your world is overrun with various horror characters
Mutant Monsters - Adds stronger versions of vanilla mobs
Pale Garden Backport: The Garden Awakens - Adds the pale garden and associated features into 1.20.1
Savage & Ravage - Adds new Illager variants and structures
Serene Seasons - Adds seasons. Configured so you can play in an autumnal world
Sleepy Hollows - Adds a spooky new biome with new mobs and structures
The Graveyard - Adds spooky new structures and mobs
Undead Unleashed! - Adds spooky new mobs