We were away in RL. Sorry we didn't make the range of Slavómir's darkness. It covers the entire city and a decent portion of the countryside around it. Enough to make it feasible for an entire army to sneak up on it. The only reason that relic power is possible at such range without being ridiculously OP is that he can't do anything with it except create darkness/shadow. There's nothing about it that can directly harm anything else.

We'd put its range at around a kilometer or so outside the city. Enough distance to make it hard to see the edge, but not so far as to affect more than a city and its rough environs. To cover such large area has taken him years to master. Many, many years. 
The edges of the artificial night are sharp. While inside its range, the area outside looks like it has night too, but if you step outside, that is no longer the case. However, if you're outside, the inside still looks dark. It would basically look like a massive hemisphere of darkness if observed from the outside. There's nothing stopping anyone from wandering in or out of the darkness. It is merely dark, nothing physically tangible.