If the monsters chosen for an open group are all defeated before a reinforcement condition take place, for example, "At the start of each Overlord turn, he may place 1 monster from the open group on the entrance." Is the Overlord then allowed to reinforce even if there's no more active monster from that group on the map? One player from our group is convinced that the Overlord can't spawn new creature from a depleted monster group, but we can't find the source of his interpretation.

Thanks in advance!

Reinforcement generally has nothing to do with the number of monsters on the board. Unless the encounter specifically says something else, you can put any monster down the encounter allows, whether or not you have monsters of that type on the board.

With the obvious exception of group limits, flightmaster is correct. Generally there is no rule about a group being "eliminated" from an encounter. Maybe he is thinking of Kobolds and the "spawner" ability?

Again, thanks for the quick response.

Thia might be a good time to have the "Your objective as heroes is NOT necessarily killing monsters" talk.