Blizzard has confirmed that during the first kill of the day, players have an increased chance to receive Rare Loot from the Loot-Filled Pumpkin, which also increases by the amount of additional Hardmode Curses players have.
Our friends on Wowhead have a detailed summary of how to enable the Hardmode before the Headless Horseman encounter.
To enable the hard mode for the Headless Horseman encounter, you must talk to the Wicker Men located just to the left when you zone into Scarlet Monastery. You can talk with each smaller Wicker Men to get one of the curses, or the big ominous Wicker Man in the middle to get all 4 curses at once!
Each curse adds an additional ability to the encounter that requires you to either dodge new obstacles or execute mechanics. Activating hard mode applies a PERSONAL debuff to you only, so you can take on the challenge by yourself and not affect any other party members.
Once you curse yourself, you will have a debuff showing what curses you have active.
Failing to dodge a curse ability or execute the mechanic will result in the cursed target getting a stack of Wicker Man’s Shadow. Getting hit once or twice is usually not that big of a deal as you only use a percentage of your health, but getting hit too much will make surviving a lot harder…
Surviving is also key – The Wicker Men will protect you from dying, but in doing so, you will lose their curses – And any additional rewards.