The Elder Scrolls Online’s Halloween event, The Witches Festival, returns on October 21st and will last until November 2nd.
To participate in the event pick up the quest “The Witchmother’s Bargain” from the Crown store. The reward is the Witchmother’s Whistle memento which will grant users a two hour 100% XP buff.
Bosses will also drop Plunder Skull containers which contain various spooky rewards. For a full list of rewards, see the official post on the ESO site.