Biomes

Each biome has it's own monsters and it's own level range. This chart helps indicate where you should train. During a bloodmoon, if the player is less than Level 20, the monsters will be 5 levels higher, but if the player is higher than or equal to level 20, monsters will be 10 levels higher, That is only valid on version 4 or lower of the mod.

The monsters on a bloodmoon will also have their status multiplied by 120%.

Deep biomes are triggered according to the world surface, multiplied by 24.

Or in layman's terms, when the lava begins to appear.

Tips
Avoid trying to fight monsters which are much higher than your level, however if you feel that it should be done, feel free to do so.