Mad Cyclops appears in the Harabah Block of Tartarus, from floors 202-213. It will be alone or in a group of two. During battle, the Mad Cyclops will alternate between using Bufudyne and Agidyne, while occasionally attacking physically.
Mad Cyclops can be found in Chapters 1, 2 and 4 of the Void Quest within the Midnight Channel. Upon being defeated, they may drop the Steel Thorn material. Selling 6 Steel Thorns at Daidara will unlock the Kaiken daggers for Yosuke Hanamura.
In Golden, Mad Cyclops appear in Chapter 1 through 4. It may drop Lepidolite for trade at Shiroku Pub.