This is a modded 1.12.2 server called End of Days. It drops you into a harsh post-apocalyptic world where nuclear war has wrecked everything, and now it's down to you and the other survivors to make it through.
Expect zombies, mutated creatures, toxic radiation zones, and plenty of danger while you scavenge for resources, build fortified outposts, and try to stay alive. Team up with other players if you want, trade, fight, or just try to carve out a safer corner of the wasteland.
It's a survival-heavy world with factions, PvP, PvE, and an economy angle, so there's room for both cooperation and conflict. If you like bleak settings, modded survival, and the whole rebuild-society-from-the-ashes vibe, this one fits that mood pretty well.