When a creature is completely surrounded by enemies that have hands, or is completely surrounded by enemies that have hands except for squares it can’t safely move to (like walls, or water that it would fall into), every attack made against that creature is an automatic critical hit. This is to make it more difficult to just wade through a sea of monsters while ignoring most of their hits, and incentivize having pets to surround an enemy with.

Possibly, do a lesser version of this (critical hits only some of the time) when a creature is almost but not completely surrounded. Though it should probably require a certain amount of adjacent enemies or something; two opponents facing off in a straight, empty passageway shouldn’t count as “almost completely surrounded” by the walls.

Several issues have been pointed out with this system if implemented naively: getting “surrounded” by two monsters in a corridor or being surrounded instantly with summon nasties or a cursed scroll of create monster could pretty much spell instant death. The original idea was that being surrounded basically amounts to being held down so that all hits have maximum accuracy.