Ring of criticality, a charged ring that has an (enchantment)% chance per hit of changing the result of the die roll for that hit into a critical hit that deals double damage (and will trigger effects such as Vorpal Blade decapitation). If negatively enchanted, it has the same chance of changing the die roll into a critical failure that deals zero or 1 damage.

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.

Change Vorpal Blade to do very high damage (on the order of x3 or x4) when it critical hits, instead of decapitation, in order to remove YAAD potential in monster hands. The new critical hit message is “Your Vorpal Blade goes snicker-snack!” (Some other message might be needed to replace the existing message when you are neutral crowned but already holding Vorpal Blade).