The Ultimate Killing Monster of Death would be the ultimate monster for killing and causing death.
Base Attack: +200
Possessions: 589 Quadrillion Gold; Rejuvenation Potion of Death +/- 2000; Morning Star of Healing +10; Scroll of Unending Orgasms
Nobody knows what the Ultimate Killing Monster of Death looks like, because nobody alive has ever seen it. The Ultimate Killing Death Monster likes to kill everything that is alive, and even some things that aren't, by crushing them in it's giant claws. Except nobody knows that it has claws becaus nobody alive has ever seen it. In fact, if you look at the Ultimate Killing Death Monster of Monsterous Death, you will die instantly.
If the Ultimate Killing Monster of Death looks at you, you will die instantly.
If the UKMoD thinks about you, you will die instantly.
If the UKDMoD makes a peanut butter and jelly sandwich, you will die.
If the Ultimate KM of Death turns the radio on to FM 103.7, everyone in the whole world and every dimension dies.
If the Ultimate Monster of Killing and Death and Dying doesn't do anything, everybody still dies.
If someone kills the Ultimate Final Mega Killing Monster of Monsterous Death and Destructive Destruction Zero, they instantly get an infinite amount of experience (which carries over with each new level). They also get whatever weapons they want, whatever slaves they want, whatever items they want, and whatever clothes they want. Then they become the next Ultimate Killing Monster of Death and Disasterous Destructive Deadly Deathly Destruction. Then when that happens, everyone in their party dies instantly.