One Life Hardcore (1lhc) is a game mode similar to Hardcore except the account is only allowed one life -- meaning dying in combat removes the status and sigil and downgrades the account to a regular hardcore account.
Warning: this mode is very RNG heavy. It is not recommended without a decent knowledge of the game's mechanics and how to circumvent this RNG.
There are exceptions to the one life rule. Lore, quest, and inevitable deaths do not count against you -- i.e. the Tank boss, Fire Witch without reflect, and similar fights. If one is missed, contact Smitty to submit it as a bug and to get your 1lhc status reinstated.
As of November 29th, all quest deaths are now exceptions to the one life rule. This includes all quest bosses (Thief Leader, Ent Boss, etc).
On December 8th, a new side quest meant for One Life Hardcore called Teleport Crystal was added. This is an item that allows the player to escape from combat once a day (24 hour cooldown) by pressing the "T" keybind (can be disabled).