What Number Mean In Durum Isles

in the basement floor of durum isles there's a item frame with a dark grey / black glass pane with the name "20"

 

There's a boss altar at durum isles. Some (all? I'm not sure.) boss altars have a number hidden somewhere near them. This number indicates the amount of a specific ingredient dropped by mobs near the boss altar is needed to use it to teleport to the boss.

In the case of Durum Isles, you need to put 20 leather in the boss altar to activate it.

 

oh cool i didn't know what boss altars did either so 2 birds with 1 stone, thanks