How to Crimson Mimic?

[Nasty Peep]

I made a new world and killed WOF to turn on hardmode, then proceeded to put a key of night in an empty wooden chest. Nothing.

Am I missing a step or requirement?

[Literally Me]

You might need to kill WOF again. There was a bug that prevented hardmode to be enabled after killing WOF even when the hardmode message was displayed. It happened to me.
If that doesn't work, you can always try the official Terraria wiki.

[Captian Fish]

one thing that might help is turning off autopause if you have that on. seems to glitch out the keys and cause them to fail.

[bloodychewtoy]

i looked on the wiki and it said autopause must be disabled

[Nasty Peep]

Oh phew, thanks guys