The Terraria song in that Roblox FNF remix is likely a Terraria Calamity OST track , and the exact one depends on the version you saw. From the results, two likely matches are “Fly of Beelzebub” and “Stained, Brutal Calamity” in Basically-FNF-Remix on Roblox.

Likely track

  • Fly of Beelzebub — used as the Plaguebringer Goliath theme in a Roblox FNF remix video.
  • Stained, Brutal Calamity — used as the Supreme Calamitas theme in another Roblox FNF remix video.

How to tell which one

  • If the remix has a faster, more aggressive Calamity-style boss theme, it may be Fly of Beelzebub.
  • If it sounds like a dramatic late-game boss track, it may be Stained, Brutal Calamity.

Extra note

There are also other Terraria-themed Roblox music IDs and remixes online, including a Terraria Theme Song asset and other Calamity tracks, so the exact song can vary by map or upload.

The most likely answer is that it’s a Terraria Calamity OST remix , especially “Fly of Beelzebub” or “Stained, Brutal Calamity.”