Hardcore

An opt-in status, taken one character at a time, where a single death normally ends it. A hardcore character carries a yellow [HC] in front of its name, and is in one of three states: Active while it lives, Falling in the minutes after a death, Fallen once the status is gone.

Fallen is permanent, and it is the status that ends, not the character: you keep playing, keep the levels and keep everything you own.

Taking it, and giving it up

You can become hardcore up to level 3, and not while you are between levels. Above that the answer is to level down and ask again.

While Active you may hand the status back, and you are warned before it happens: you can only take it again at level 3 or below. While Falling you cannot hand it back, and once Fallen you cannot take it again on that character at all.

When a hardcore character dies

The moment you die the status leaves Active:

Falling, if a Pure Balm could still be used on you - see the limits below. A clock appears in the top-left corner counting down what is left.
Fallen, straight away, if no Pure Balm could reach you.

A kill by a DM is not counted against you.

The window is ten minutes from the death, and it keeps running whether or not you stay online. Inside it, only a resurrection that costs nothing restores you to Active - a Pure Balm, or any of the other free resurrections listed on Resurrection. A resurrection you pay for brings you back to life as usual, but not to Active. When the ten minutes are up, the status is Fallen.

Deaths, level 20, level 40, taking the status and handing it back are all announced.

Pure Balm limits

A Pure Balm can only be used on a hardcore character of level 20 or above. Below level 40 a character may be saved by one; at level 40 up to five in total, counting the one already spent. The balm itself can be used once every 16 hours.

What being hardcore changes

PvP is closed between the two kinds. Outside the arena, a hardcore character cannot attack a non-hardcore one, and a non-hardcore character cannot attack a hardcore one.
The XP bank is closed to you. A withdrawal is refused; a deposit takes the experience off your character and stores nothing.
Loot and Vengeful rolls come up more often in a dungeon. If everyone registered in the instance is hardcore, both chances are doubled. In a mixed instance each hardcore character adds 20% to the drop chance, to a ceiling of 80%, and 10% to the Vengeful chance, to a ceiling of 60%.
A dungeon run is shielded from DMs. In a level 40 or raid instance holding at least one hardcore character, only an admin can teleport in.