Theft, extortion, drug trafficking, smuggling, assassinations, prostitution,
Depends on the cultural background of your fantasy world. For prostitution one may imagine lots of euphemisms such as:
- Flower Dealers/Flower bearers/seed bearers/basements florists (as in the flowers are euphemisms for prostitutes)
- Fruit pickers (same as flowers)
- Night walkers (mesh of streetwalkers and Lady of the night)
Drugs - heavily depends on the effect the drug will have and what context people take them
- Dream keepers/dream guides/dream catchers/dream granters - things like opiates
- Candy makers/night confectioners/sugar barons - if the drugs are brightly colored
- Euphemisms for drugs: slum sweets, proles peppermints, poor man's pepper upper
- Joy bringers,
And if you are lazy, just look up euphemisms for these things and mesh some of them together. Tbf, more innocuous names make things a lot insidious and scary and stuff. Plus it will be a great way to cover up their crimes (like drug dealers used to do with adopting minecraft lingo) (For example if the god of the death is called batum, then the guild may be called; Batum's messengers, Batum's followers, Batum's blessed, Batum's children, Batum's emissaries)
With assassination: what kind of things does your fantasy world associate with death? Does it have angels, do people believe in the afterlife? If you know, then it's very easy to make very unique guild names appropriate to the setting and you will probably not need people's help anymore :).
You could also look at existing names of gangs and maffia (name it after the founder or something of the country of origin). It also heavily depends on the culture within the guild.