Sigils or Chat Sigils are small icons that appear next to your name in Chat. You can only have one sigil active at a time, which can be set with the 'Set Sigil' button in the chat window. There are four types of sigils: Skill Sigils, Monster Sigils, Holiday Sigils, and the Hardcore Sigil, each of which are obtained in a different manner. Sigils should not be confused with Chat Tags, which can be used at the same time as sigils and are obtained by donating to the game.
Holiday Sigils are obtained by completing special, which are released as special one-off events for a limited time around holidays. The three holiday events are Halloween, Christmas, and Easter, which occur in October, December, and March/April, respectively. New holiday sigils are released every year, and the previous holiday sigils can NOT be reobtained in subsequent years, however previous holiday sigils are usually made tradeable only during the release of their respective new holiday event.
The Hardcore Sigil is available on any Hardcore account. It cannot be used at the same time as other sigils, and can be toggled on or off just like the other sigils. There is no way to obtain the Hardcore Sigil on a regular Softcore account.