Mica

In the depths of a dark, foreboding cavern in the heart of the Undervale lies a giant underground fungus known as "Mica." This massive organism is said to be one of the oldest living beings in the world, with tendrils that stretch for miles underground, and it is rumored to hold the souls of the damned.
Mica is a fearsome sight to behold. Its massive cap stretches high overhead, reaching up to thirty feet in height, while its thick, gnarled stalks anchor it firmly to the cavern floor. The top layers of the fungus can be harvested by the daring and the foolish, but otherwise, it is impervious to non-magical forces.
Mica is home to colonies of myconids, who live in small hidden settlements, (and some large, thriving colonies) nestled underneath its sprawling tendrils. These creatures are fiercely territorial, and they will defend their home against any who would seek to harm it. They are known to be expert craftsmen, fashioning weapons and tools from the sturdy stalks of their fungal home.
Mica has a number of other strange and unsettling traits. Its flesh is a deep, purplish-black color, and it exudes a thick, viscous sap that is sought after for its magical properties. The fungus emits a low, rumbling hum that echoes through the caverns, and those who hear it claim to feel an unsettling presence nearby. Its tendrils are covered in strange, glowing shapes, suggestive of the shapes of Dwarven runes, that seem to pulse with a dim, unearthly light.
The depth of Mica in the caverns and tunnels of the Undervale, however, is part of the reason suggested for one of Mica's known traits. When harvested pieces of the great fungus are taken above ground, they have a severe reaction to natural light. If exposed to sunlight, the fungus withers and shrivels instantly, as though burned by the touch of the sun.
Despite its fearsome appearance and rumored dark powers, Mica is a vital part of the ecosystem of the caverns. Its roots bind the soil and stone together, preventing cave-ins and stabilizing the labyrinthine tunnels. And, for those brave enough to harvest its top layers, Mica can provide valuable sustenance and magical properties – many, as yet, undiscovered.