Hallow's End is arguably one of the most important holidays to do on every toon.
You can only set up a toon with access to the Alemental costume once per year, during Hallow's End. Once you set it up, its available year-round.
https://www.wowhead.com/spell=190968/mysterious-brew
How to get the Alemental costume:
1. Be at least level 100.
2. Get a level 3 WoD garrison on the toon.
3. The Hallow's End holiday must be active. (October).
4. Talk to Orukan in your level 3 garrison and grab the dailies.
https://www.wowhead.com/npc=96705/orukan
5. Complete the dailies to earn 5x Spooky Supplies
6. Talk to vendor Izzy Hollyfizzle in your level 3 garrison
https://www.wowhead.com/npc=96362/izzy-hollyfizzle
7. Buy the Witch's Brew (garrison decoration) for 5x Spooky Supplies.
https://www.wowhead.com/item=128663/witchs-brew
8. Right-click on the Witch's Brew to apply it to your level 3 garrison.
9. Brew-Witch Shadowpaw will spawn in your level 3 garrison.
https://www.wowhead.com/npc=96259/brew-witch-shadowpaw She is at 47,54 in the Horde garrison and 43,46 in the Alliance garrison.
10. Next to Shadowpaw is a cauldron of brew.
Now the rules get strange.
11. If you hover your cursor over the cauldron, your cursor will turn into a gear. Click it and you will get an item called Mysterious Brew. This Brew is like the Hallowed Wands. You can use it to change anyone into an Alemental for 5 minutes. It can be yourself, a party or raid member, someone of your faction that you are not grouped with at all. It cannot be used on NPCs. The Brew has 5 charges and is unique. You cannot carry more than 1. The cauldron is only clickable if you were not carrying a brew when you entered your garrison zone and have not used it in your current visit yet. If the cauldron is not clickable, make sure you delete the brew from your bags, then do a quick zone out / zone in of your garrison and the cauldron will be clickable again.
The garrison decorations can be put on display year-round. So once you add this decoration to your garrison, you can use it 365 days of the year. You can only unlock it during Hallow's End, and it must be unlocked on every toon you have. It is not account-wide.