Phosphorous is showing off "her" might by extending its claws. Phosphorous is indeed a dark lord of the abyss--besides the nine eldritch lords!
Inspired song is "You Should See Me in a Crown"

Phosphorous stands for minutes on a lonely hill nearby Paul's house thinking about the eldritch lords she once fought against. She fought them before with the light and failed. Now she will do it again in darkness--even if it means recruiting the creatures of the night! No nightmare beast will take the determined dutchie down!

"Count my cards watch them fall;
Blood on a marble wall
I like the way how they scream!
Tell me which one is worst;
Living or dying first,
Sleeping inside a hearse
(I don't dream)
You say, come here baby
I think your pretty.
I'm okay but I'm not your baby;
You think I'm pretty?
Then you should see me in a crown!
I'm gonna rule this nothing town!
Watch me make them bow
one by one by one;
one by one by one."

Song and lyrics belong to Billie Eilish.

It matches Phosphorous's personality so well!