Chinese slogans typically come in idiomatic (four characters) phrase
if "Forces of Evil, Come and Obey" means "we are the forces of Evil, come and obey us" then I suggest:
"(the power of) good has declined and (the power of) evil has grown; those who submit to us flourish"
jyutping: /dou2 siu1 mo1 coeng2 seon6 ngo5 ze2 coeng1/
phonetic: "doe siu mor zhung, Shun wu jer chun"
And it rhymes
If "Forces of Evil, Come and Obey" means "you, the forces of evil, come and obey me' then I suggest:
"道長魔消，逆我者亡" (the power of) evil has declined and (the power of) good has grown; those who resist us perish"
jyutping: /dou2 coeng2 mo1 siu1/, /jik6 ngo5 ze2 mong4/
phonetic: "doe zhung mor siu, yiey wu jer mong"