Board Thread:Help Desk/@comment-7628100-20131016230701

When I was playing on my hardmode character today, i saw 'the air around you grows colder' in green text on the 5th night in in hardmode. one minute later, skeletron prime suddenly spawned. the only suff i have is: one beam sword, zealous minishark with 82 silver bullets, full molten armor, a piranha gun, and a ruthless dark lance. Skeletron prime took out my wizard in like 2 seconds, and then me while I had a piranha latched onto his saw arm a moment later.

I have these questions about this and I'd like them all answered:

Why did this happen. Is this a bug, or was I just seeing things. Is there a specific interval of nights he spawns if it wasnt a bug? 