Subterranean Sentinel Shadow Striker Deep Dweller Cavern Crusader Undercity Guardian Tunneler Mole Man (if villainous) The Mole Nightcrawler (if appropriate for the character's abilities) Earthshaker Magmafist Rockbreaker Sewer Serpent Midnight Prowler The Dredger The Abyssal The Subterranean The Underking (if villainous) The Deep One The Groundshaker The Tunneler The Shadow Weaver The Silent Watcher The Unseen Protector The Hidden Hand The Earthborn The Chthonian The Geomancer The Tectonic The Tremor The Quakebringer The Subterfuge The Cloaked Figure The Enigma The Wraith The Phantom The Specter The Ghost The Shade The Night The Dark The Void The Abyss The Depths The Underworld The Netherworld The Shadowlands The Underdark The Deep The Lowlands The Undercity The Subterranean City The Hidden City The Lost City The Forgotten City The Sunken City The Ruined City The Desolate City The Abandoned City The Ghost City The Silent City The Empty City The Dead City The Cursed City The Haunted City The Shadow City The Dark City The Night City The Underground City The Hidden World The Secret World The Lost World The Forgotten World The Unknown World The Mysterious World The Enigmatic World The Shadow World The Dark World The Night World The Underground World The Subterranean World The Hidden Realm The Secret Realm The Lost Realm The Forgotten Realm The Unknown Realm The Mysterious Realm The Enigmatic Realm The Shadow Realm The Dark Realm The Night Realm The Underground Realm The Subterranean Realm The Hidden Dimension The Secret Dimension The Lost Dimension The Forgotten Dimension The Unknown Dimension The Mysterious Dimension The Enigmatic Dimension The Shadow Dimension The Dark Dimension The Night Dimension The Underground Dimension The Subterranean Dimension

These names offer a variety of styles and tones, from heroic to villainous, allowing for flexibility in character development. Consider combining elements from different names to create something truly unique.