Angus was a local drug dealer who was known to the City Guard, well, at least to Jethro. He was a shady character who can usually be found standing on the corner near The Hairy Bear, the kitty-corner from the Eager Thighs brothel, or in the slums of the Temple Quarter.
Geralt must find Angus and get from him information about the Salamandra drug trade in Vizima. It is from him that Geralt gets the information to locate the Salamandra hideout where the manufacture of fisstech seems to be based. He can get this information either by paying the dealer for a letter of recommendation in order to enter the drug lab with minimal fuss or taking that same letter from him by force. Alternatively, our hero can simply enter the lab using force and dispense with the letter altogether.