When can I play in Hardcore mode?
Solution 1:
You must reach level 10 with any character before Hardcore is unlocked. You do not need to keep a character at level 10 however, as deleting all of your level 10 softcore characters will not remove your ability to play hardcore.
Quoth Blue:
Level 10 is when we open access to create hardcore characters. We want players to have a general idea of how to play before giving hardcore a try. Experienced players will easily reach this level in a few hours, so we think this makes for a good balance.