Muffet

"Muffet pours you a cup of spiders."

-Text box quote

Muffet
Muffet is a tall spider-humanoid monster featured in the game Undertale.

In battle
In battle, Muffet will change the color of your soul to purple, which then sets your heart on three strings which you can move around on. Her next attack will be shown by a spider that walks in from the right side of the screen She most commonly will attack by putting spiders along the strings, throwing croissants across the strings (which return back after being thrown) and letting her "pet" spider go rampant and pull the strings down, while she puts spiders as an obstacle.

How to spare
In order to spare muffet, you can either use a spider item from earlier in the game, or if you previously purchased one item from the beginning she will eventually let you go. If you bought one of her items, she will outright not battle you.

Origin
Muffet was designed by "avasdemon" on twitter.