Humor are slime creatures which spawn in the underground, fairly frequently in Bethesda Susa and occasionally in the asphalt mines. There are four types of humor, one for each of the four Elemental Damages.
Humors spawn with four randomized pseudopods of same type as its element. When a humor dies, it explodes with effects related to its elemental type. For example, when a humor with 'hoary' (i.e cold) pseudopods dies, it causes the temperature around it to drop rapidly on death. This has the potential to freeze anything nearby.
Humors with acidic pseudopods drop pools of acid on death. Humors with flaming pseudopods cause a fiery explosion on death.
Some humors toggle between the four elements every 50 turns or so.