Don't know about terraria specifically, but it's a sliding scale of how you like beef cooked right?
The mid section is the usual meat doneness from Well done to Rare
Going up it parodies loot rarity types from Rare to Mythic (maybe Unreal is one too?). The idea is that, going "more rare" than rare beef is
- Rare - Rare beef
- Epic - raw beef
- Legendary - Live cow
- Mythic - All the ancestral cow-like creatures that evolved into cow
- Unreal - The primordial ancestor of all life on land
They talked about emptying the catacombs at the start, so I assumed it was previous Night Sisters.
I seem to remember from Clone Wars that the Night Mother has some kind of connection to all the previous Night Mothers that came before.
Maybe Night Sister WiFi only goes so far, so you have to bring the router with you.