Diablo Wiki

Keywardens are four unique demons in Diablo III.

Their name comes from their original role: sometimes, upon death they would drop a key for the Infernal Machine event. As of patch 2.3, they drop Infernal Machines of corresponding type instead.

At levels 61-70, all keywardens have a small chance to drop any level 70 device, which does not compete with their chance to drop their primary machine. This allows any keywarden to drop up to two devices upon death at levels 61-70. At level 60 or higher, they will be marked with a boss icon on the map, and an arrow will point their location.

As of patch 2.4, their name also shows the material ultimately harvested from bosses in the realms for which they drop their machines.