Kill the rat god.

Kill the rat god is a side quest in Fallout 2.

Quick walkthrough

 * 1) Talk to Slim in Trapper Town (Klamath).
 * 2) Obtain the key to warrens from Slim via barter or stealing, alternatively pick the lock.
 * 3) Enter the Rat Caves on the northern part of Trapper Town.
 * 4) Kill the Rat God on the second sub-floor.

Detailed walkthrough
There's a big problem in Trapper Town, the west end of Klamath. Slim tells you he thinks there's something in the caverns below controlling the rats that have been infesting the place.

You can get the key to the warrens door in one of several ways, including:
 * Convincing Slim to give it to you
 * Stealing the key from him
 * Picking the door lock
 * Sleeping with Slim (Female character only)
 * Killing him and taking it
 * Feeding the dog near Klamath's town center a piece of meat jerky (he'll cough it up and follow you around)
 * Clicking barter, then trading for the key (it has a value of 0, so just add $1 to your side and he'll accept.)

At any rate, once you get into the warrens, battle your way down through the caves until you reach the second sub-floor. There you'll face Keeng Ra'at the rat god, the brains behind the invasion. The giant rat has a very large amount of HP, and is a difficult creature to kill, especially since you will most likely be at a very low level when you come across it, and with very weak weaponry. Still, if you hit it with a Critical Hit to eyes or to groin, no matter the damage - the rat will be always killed.