Like people went through this portal and had to declare their profession and powers. Also I feel like the protagonist killed two vampires in the beginning, and there was like a "good" faction and a "bad" one. And then there was this woman who didn't remember who she was, but she was important, and the boss had something to do with her.
This is (the first book of) Sergei Lukyanenko's excellent Night Watch series.
From Wikipedia (relevant parts only; stress mine):
In the story's worldline there exists a magical realm beneath the surface of all things — referred to as the Twilight. The Others were the humans from long ago who figured out how to step into the Twilight. The aura of any Other, or emotional state at the time of their first entry into the Twilight, determines whether or not the Other will become a 'Light' or 'Dark' Other.
A reluctant mage recently reassigned to field work, Anton Gorodetsky tracks vampires by drinking blood and channeling them. Anton finds the vampires who have been calling Egor with their power and, as they don't have a special license, kills one of them, while the other (a female) gets away.
He returns to the Night Watch headquarters, where his boss, Boris Ignatievich, informs him that he could be in danger as Zabulon (head of the Day Watch) might want revenge for his actions in killing Dark Others and gives him a stuffed owl called Olga (who later transforms into a woman), for his protection. He then discovers that Olga can speak and is a sorceress trapped in an owl's body as a punishment.
It is later revealed that
Olga is "the boss's lover from way back" (quote).
It's a fascinating book series and well worth reading.