First discovered by parabotanist Avalon Faryonda, this mushroom was unfortunately named by her apprentice, who had woefully misunderstood the fly agaric's etymology. Despite Faryonda's best efforts, the mushroom's name found its way into the Royal Botanical Encyclopaedia, Fifth Edition.
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You can take a Bonus Action to consume this mushroom and gain 15 Temporary Hit Points that last for 1 hour. While you have these Temporary Hit Points, you have the following benefits:
Your size changes to Large if you're in a big enough space. Anything you wear or carry changes size with you until you drop it.
You deal double damage to objects and structures.
Your High Jump's distance increases by 20 feet and you subtract 20 feet from the distance fallen when you calculate fall damage.
You can take a Magic action to perform a High Jump without provoking Opportunity Attacks, landing in an unoccupied space within 20 feet of you. After you land, each creature within 5 feet of you that is smaller than you must succeed on a DC 13 Dexterity saving throw or take 2d8 Bludgeoning damage. A creature that fails the save by 5 or more also falls Prone.
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DESIGN COMMENTARY
Next in this batch of size-changing items is a mushroom inspired by arguably the most famous size-changing character. This mechanic is based on Temporary Hit Points. For a consumable Uncommon item, 15 Temporary Hit Points is equivalent to a level 3 Armor of Agathys spell, which would be an Uncommon 3rd-level spell scroll. Instead of damage, this item grants some ribbon abilities (double damage to objects doesn't come up often and you can usually use alternatives to cover the leap) plus a damaging ability that more or less equates to taking an Attack action (~10 average damage).
Thematically, it was important that this mushroom lets you 1. Jump very high, 2. Break previously unbreakable blocks, and 3. Potentially "flatten" an enemy. Note that the High Jump distance is a flat increase, so it'll increase your standing vs running jump by the same amount. Boing!
Visually, I wanted to keep this as simple and recognizable as possible. The only difference of note is the spiral stems, which are meant to look like springs - boing!!