Underblossom is an AU where monsters' emotions and their entire lifeforce/soul is represented through flowers. These flowers grow out of a monster's body and clothes and reflect what they feel and experience. Monsters who have a more positive outlook on life and are in a good place emotionally "blossom" more frequently, while those who are more pessimistic bloom less often. Monsters can also bloom intensely when they experience large spikes in emotion, negative or positive.
These flowers always glow, but the vibrancy of that depends on the monster they are connected to. The flowers themselves are made of magic, and any petals that fall usually fade away. Frisk carries around a bag with them full of flowers they've collected from various monsters, having asked them to blossom for them. Each monster has a different flower that applies to them, all with their own specific meanings that represent each character.
This is a broader AU that can apply to any Undertale/Deltarune canon or AU.
"In Underblossom, monsters are affected by their emotions and flowers bloom depending on how they’re feeling. A lot of blossoming shows that a monster is healthy and happy, because flowers tend to bloom most often when a monster experiences positive emotions such as happiness, love, excitement, etc. and also when they get really flustered or embarrassed!!
Flowers can also bloom when monsters experience EXTREME spikes in emotion—this can be any emotion, even negative ones.
All monsters have flowers!! Everyone with magic. (So it doesn’t apply to Frisk.) So as for humans, no, they cannot bloom their own flowers! The flowers come from monsters’ magic and is a reflection of their soul–it is said that every living being in the world has a “CORE” flower that is entwined with their soul and is what gives them life. So, while Frisk isn’t able to bloom their own flowers, they do have a CORE flower inside of them!
Monsters can try to lie, but their flowers always betray them since they reflect their most honest and truest emotions–hence why the majority of the monster population are all so open with their feelings. Hiding feelings is difficult and puts one’s body at risk so they don’t attempt it often.
Since flowers are directly connected with a monster’s soul, they’re fused with their attacks as well. So Sans’ Gaster Blasters and Papyrus’ bones, for example, have their flowers growing out of them as well ^^ So Undyne’s and Asgore’s spears have flowers growing out of them…Toriel’s fire comes with a whirlwind of petals…stuff like that! I don’t think the flowers themselves could injure Frisk in battle, more that they’re just an accessory which further reflects a monster’s soul!"
-Shima, the creator of the AU
After a monster dies, it is said their CORE flower is left behind, the only remaining physical artifact proving they existed. Many monsters decorate the graves of their loved ones with their CORE flower, or preserve their CORE flower in their homes. Monsters never touch or interfere with other CORE flowers—they are said to be one of the most sacred magical things, and not even the most wicked of monsters would dare insult the dead by tampering with one.
- Flower meaning: Rememberance during partings or after death; a connection that lasts through time; loyalty in a relationship, despite separation or other challenges; reminders of one's favorite memories or time together with another person; and growing affection between two people.
- Sans, being a monster that is more prone to undergoing negative emotions due to the constant RESETs, blossoms much less frequently than anybody else.
- Flower meaning: friends, endurance, daily rememberance, goodness, and lasting affection.
- Papyrus is considered to blossom most vibrantly and frequently out of all of the other monsters due to his overly positive attitude and happy outlook on life.
Undyne: Green gladiolus
- Flower meaning: honor, rememberance, strength of character, faithfulness, sincerity, integrity, infatuation, and never giving up.
- Flower meaning: Creativity, inspiration, renewal, vitality, awareness, inner reflection, memory, and forgiveness.
- Alphys is on the opposite side of the scale of blooming, second to Sans in the small amount she produces because of her inner war.
- Flower meaning: Purity, silence, devotion, caution, serenity, grace, and calmness.
Asgore: White hyrdrangeas
- Flower meaning: Heartfelt and honest emotions of any kind; gratitude to someone else; developing a deeper understanding between two people; heartlessness and acting without thinking about the feelings of another; abundance and prosperity; and grace and beauty.
- Flower meaning: Long life; feelings of adoration, admiration, and platonic love towards a person, such as a family member or friend; loyalty and strong bonds between two people; seeking out positivity and strength; nourishing oneself and others; brightening one's mood; and good luck and lasting happiness.
- Flower meaning: Taking care of yourself and your family; temperance; emotional evenness; passion that is still developing; elegance and wealth; femininity and feminine beauty; and abundance, especially of beauty or intelligence.
Napstablook: Silver alliums
- Flower meaning: Unity, patience, humility, prosperity, and good fortune.
Gaster: Black hellebores
- Flower meaning: Usually associated with demons and possession, it is a poisonous flower. However, they are also used as protection and to drive away bad atmospheres.
- It is said that Gaster's CORE flower still exists in the void somewhere.
- Flower meaning: Blossoming friendship that is still in its first stages; reminiscing about pleasant memories shared with a friend or relative; reaching one's full potential and achieving their dreams; existence throughout eternity, and extending one's time with what they love; and everlasting love.
- Flower meaning: Warnings about future misfortunes or challenges; dark and unpleasant thoughts that distract one from their happiness; being cautious about new situations; harmonious communications between friends and family members; gratitude for a favor from someone else; standing out from the crowd; and justice and peace.
Undyne: Green chrysanthemums
- Flower meaning: Lasting friendship and non-romantic affection; support from one's family and loved ones; cheerfulness and good spirits; rest and recovery after a long trial or challenge; enduring life and rebirth; and loyalty and devotion, both romantic and platonic.
- Flower meaning: Despair and grief over the loss of love; beauty and warmth of the rising sun; winning the affections of someone through hard work; creativity and the drive to succeed; cruelty and coldness due to jealousy; and promoting cheer and good relations in a relationship.
- Flower meaning: Protection against evil and ill wishes; forsaken or forgotten love and affection; anticipation and excitement for something in the future; and the death of a loved one.
Asgore: White wisterias
- Flower meaning: Good luck; welcoming someone to a new town or home; celebrating the youthful vitality of a young friend or child; serious devotion; and new births.
-Frisk is said to have a rose as their CORE flower. This changes color depending on the strength of Chara's influence. A white rose represents purity. Red is tainted with blood, and black is when Chara has completely taken over and wilted Frisk's heart.
None of the other flowers have been decided for any other characters/AUs. Anyone is welcome to pitch flower/character ideas, as long as the meanings make sense.