Mesmetron

Mesmetron is a Fallout 3 weapon that is used in the quest Strictly Business, and is given to you by Grouse (the Paradise Falls entrance guard) when you first begin the quest.

Characteristics
When used on the specified quest NPCs, it puts them into a hypnotic stupor, allowing the player to rob and enslave them (by putting a Slave Collar on them) and send them to Paradise Falls. It also appears to work on some other NPCs, but often simply sends them into a berserk frenzy in which they become hostile towards the player and every other NPC in the area. Occasionally, the victim will run several steps before their head explodes.

It uses Mesmetron power cell as ammo. You start out with 50 Mesmetron power cells. Grouse will also sell you additional power cells, but at a fairly steep price (200 caps for 10).

If you don't want to take the Strictly Business quest, you can obtain the Mesmetron by killing Grouse and looting his corpse. You have to kill him to obtain the Mesmetron, as it cannot be pickpocketed. If you kill Grouse, the Mesmetron can only be used for robbing or frenzying NPCs, as you cannot enslave NPCs with Grouse dead and Paradise Falls hostile towards you. You also cannot obtain additional Mesmetron power cells from anyone other than Grouse.

Background
The Mesmetron is an experimental pre-war "non-lethal pacification" weapon that was being tested in the months immediately preceding the nuclear war. It operates by sending a signal pulse into a human target's brain that scrambles higher-level mental functions, leaving the target extremely susceptible to suggestion. Still in its testing phase, the weapon was found to have the potential to trigger two undesirable side-effects... 1) extremely increased aggression, or 2) "spontaneous cranial eruption".

Grouse found it in a pre-war military installation, and gives it to the player character for field testing if they accept the Strictly Business enslavement quest from him.

Trivia

 * The Mesmetron is similar in both name and effect to the Persuadertron from the Syndicate game series. The Persuadertron, however, allows complete domination over a victim without the need of a Slave Collar.
 * Similar to the L.O.O.K.E.R. (Light Ocular-Oriented Kinetic Emotive Responses) gun, a light pulse device that gives the illusion of invisibility by instantly mesmerizing its victims into losing all sense of time.

Appearances
Mesmetron appears only in Fallout 3.