The dungeon mode in Warcraft Rumble is unlocked by progressing to 30 Sigils. The dungeon mode is effectively a survival mode in the game where your army will have to complete 3 different maps one after another. As you progress these modes will get increasingly harder and allow for more rewards accordingly into your account.
As you see highlighted above, the Dungeons give your leaders boosts for different traits which allows you to give levels to minis that fit the right trait. Highlighted in Red shows that the trait bonus is active because Abomination is an Undead unit so he gets the level bonus, as with the Dark Iron Miner because he is an Unbound mini. But the Yellow highlighted trait isn’t active because the Gryphon Rider is an Alliance mini, not an Undead mini.
So to maximise Dungeon rewards you want to fit the right traits in the right slots to get that level boost in those minis. So Lockers will likely change his team around to make the traits fit the team.
The main resource that you will gain by completing the dungeon mode in Warcraft Rumble is Arc Energy as it will allow you to level your minis to make them stronger and so they can get a trait to make them unique and it allows you to customise your teams to better suit your playerstyle. You can earn Arc Energy from clearing bosses in the Dungeons.
Dungeon maps will award you with relics. These relics can be used to buff certain units and are faction-specific. When You fully clear a Dungeon you can grant bonus levels for the minis of the Leader that you used. For example, when it is Alliance week and you clear the Dungeon with a Tirion Fordring as the leader then in your team you can give levels to minis in your team matching the traits highlighted.