Ahh, the Pits DLC. Now that is a welcome addition if you also enjoy tactical games where there is a real challenge to face!
Unlike other DLCs such as Pirates of Belerion or the Skelmar Invasion, the Pits will not give you access to a new region. Rather, using new points of interest on the base game maps, you will gain access to some kind of arena; the Pits differ from classical arenas in offering single battles, rather than a series of 3 ending with a champion.
In return for this specific game mode, each battle will be set with an unique rule: you can face your own team, you can suffer from a debuff that'll put out of combat one of your mercenaries if attacked by a kind of opponent, you can suffer terrible penalties from knockback skills, and so on - you will basically need to think again on your approach to each battle, for they all have theirs quirks. The brute force method may not prove to be always efficient: opponents are often very high in armor, health & damages. Quite a change from the groups roaming outside!
Each battle will have its own map, with traps - lots of them. They are heavily punishing, so placement is key and can play both ways, meaning it can easily play against you! One battle per location will offer a champion, sometimes on their own, sometimes with a swarm of opponents or nasty mechanics. There are various valuable rewards depending on your progress through each location as well as throughout all Pits; don't miss out on them, they include legendary gear!
Now for the small grief I'd have against it (hence four stars), it's the ever-so-dumb AI usually simply running to the closest target (except some specific targeting rules, making the Pits much more interesting). It's easy to bait opponents into traps, so if you're ready to put some of your mercenaries at risk (note that they won't die, the system is based on arenas so they walk out unscathed) several fights become much easier to deal with.
Still it's a nice addition worth its salt!