Monsters have a chance of spawning when a ruins is generated. Their difficulty is indicated by their Monster Power.
A party of up to four users total can engage a monster at once. Upon defeating the monster, loot is shared equally between every user that participated. The amount of Gold and Exp gained is significantly larger compared to exploring a ruins. The base amount of these can be up to 40x more before the reward is split.
A monster lobby is created once a monster has been discovered, and the player who explored that tile will become the owner of the lobby.
Use the command a.lobby start to start combat.
It is possible to let up to three other users join you by opening the lobby with the command a.lobby open. Others can then join by using the command a.lobby join.