Experimental MIRV
This is a unique Fat Man with 1610 x 8 damage. It is found in the bomb shelter section of the Guard Depot. All five Keller Family Transcripts (Holotapes) must be found first. It fires eight mini-nukes at once in a random spread pattern. This does lots of damage, but is essentially useless other than for entertainment value, or for leveling entire towns.
Alternately, go to Fort Bannister, and use the following video to find the Fat Man.