Sugary Soul Shards are a consumable item that gives the entire world a dark tint, while also turning the sky purple. It gives a very ethereal look, and each Sugary Soul Shard buffs the player for 5 minutes.
You can likely only get the Sugary Soul Shards during the 15 Year Anniversary for World of Warcraft, and right now we do not know if they will return in the coming anniversaries.
Anyway, you can get the Sugary Soul Shards by purchasing them from a vendor called Waiter. He is found in Caverns of Time, just next to Chromie and the Chronocrystals there. He will only be there for the 15 year Anniversary, and will disappear when this event is over. They cost 5g per stack of 5, so effectively 1g each.
Them item does not seem to be limited to the event, meaning that you can likely stack up on them if you want to, and use them after the event is over. You just won't be able to buy any new ones.