Lyrical assassin
Hysterical ass, terrible black ski mask'n
Supply for the askin

from Loki – Mass Appeal Lyrics on Rap Genius

Meaning

He’s a lyrical assassin (common hiphop phrase) is also hysterical and terrible black ski mask'n. and supplies what you need.

At first appears that he is mentioning crimes. being an assassin, black ski mask'n and “supplies what you need”.
But can be looked as…

“lyrical assassin” (kills his lyrics)
“hysterical ass” (extreme emotion in his lyrics)
“black ski mask'n” (connects with lyrical assassin and pretty sure assassins where masks)
“supplies what you need” (brings that “dope” lyrics when you need em)

So he might be saying that he has a dark side (the crime) when doing what he does best (making rhymes). Which he has said before in a previous song “ So if i keep making this sad cryptic rap shit…”

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