Mesmetron

The 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 start 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 many other NPCs (aside from quest related ones), but can simply send 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 Cells as ammo. You start out with 50 Mesmetron power cells. Grouse will also sell you additional power cells, but at a fairly high price (200 caps for 10 Power Cells).

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.

Using the Mesmetron
As previously mentioned, using the Mesmetron on an NPC can have 3 different outcomes. These effects vary and are generally unreliable, so it is advised to save before using this weapon on anyone important. Due to the fact that using the mesmetron is considered assault, it is advised to "mez" someone while not detected (Stealth Boys and the Chinese Stealth Armor will come in handy here). Be aware that mezzing anyone who is already wearing a Slave Collar will cause the collar to malfunction and explode, killing the victim. Hostile NPCs who were mezzed can also be turned friendly by being freed by the player (high Science skill required). An analysis of the Mesmetron's effects:

Mesmerize/Stun (50% Probability)
This first effect stuns the victim for a few seconds, during which they stand still and their heads sway from side to side. When the victim is stunned, it is possible to engage in conversation with them. Note that some characters cannot be stunned, in which case the probability of the Frenzy effect is raised to 80% to replace stun.

While in conversation, you will first have 3 options. The first option convinces the character to allow you to take their things (by either telling them it's a magic trick, that there is a thief on the loose, that you think they stole something from you, or by just telling them to give you their things) which will open up their entire inventory (even the items they are wearing/using) and allow you to take or even add any item. The second option slips a Slave Collar on the character (instantly netting the player negative Karma regardless of who the victim is). The final option osimply tells the victim to "walk it off"; this will exit the conversation and make the character resume what they were doing (presuming you haven't made any onlookers hostile). The second set of dialogue options (unlocked by slipping a collar onto the victim) is quite simple. The first option is the "cruel" way to inform the victim that they are now a slave ("...your head will explode!"). The second option once again allows the player to check their inventory, and the third option is the slightly nicer way to inform them of their "new occupation" ("...nothing personal").

If the character is not talked to before their stun wears off, they will become hostile.

Frenzy (30% Probability)
This second effect causes the victim to go into a state of frenzy, attacking anyone nearby (most likely the player), and causing all other characters to turn on them. The onlookers may also become hostile to the player if he is seen stunning the victim. This method can also be a reliable way to get rid of victims without attacking them or getting negative Karma.

"Spontaneous Cranial Eruption" (20% Probability)
This final effect of the mesmetron may not be the most useful, but it is the most spectacular. If a victim is affected by the mesmetron in this way, they will become hostile (and either run, stand still, or attack) for just a moment before their head literally explodes. This effect will instantly net the player negative Karma (if the victim was good/neutral) and make any nearby NPCs who were not hostile to the victim become hostile to the player.

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.

Bugs

 * A glitch may happen where the player you mezzed is still alive but you can are still able to search them. Afterwards, they and everyone in Megaton will attack you.

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.
 * The method of operation is also similar to a Star Wars expanded universe device called the Gun of Command, an EMP weapon that is used by the matriarchal Hapan nation in executions. Further, the projectile fired by it is a reference to the stun setting of an Imperial blaster rifle, used on Leia in Episode IV.

Appearances
The Mesmetron appears only in Fallout 3.