Uranium imps are a new minor demon added in UnNetHack. When a uranium imp is hit in melee, both the imp and the attacker are teleported together to a random location on the same level, so a fight with one can lead to hopping all over the level. A player with teleport control will be able to choose his/her destination, but will still be accompanied there by the uranium imp. This power is nullified on no-teleport levels.
Uranium imps don't deal dangerous amounts of damage, but their teleportation can land an unsuspecting player in a dangerous situation. Additionally, they can very easily separate adventurers from their pets.
Teleport Control isn't a perfect defense against the teleportation ability; 4/13 of the time, the teleportation will be "too chaotic to control" and the location will be random despite teleport control. The 4/13 chance is a reference to the source material for uranium imps - the number 413 keeps showing up throughout the story.
However, you don't have to fight them—if you #chat with them, they will run away for 10-30 turns. This is a reference to the uranium imp's source material, Homestuck, in which a bunch of imps are "defeated" by the main character yelling at them to leave. The source code acknowledges this reference with a comment "/* That is IT. EVERYBODY OUT. You are DEAD SERIOUS. */" before defining the running away behavior.
Despite being imbued with reality-warping powers by the Green Sun, these trouble-
making servants of darkness seem unaware of their true power, and
probably would run away if yelled at.
GG: those stupid things are impossible to kill :(
TG: no you can kill them
TG: youll get better dont worry