Mushroom Bomb

The Mushroom Bomb first appears in Finn the Human (episode) and is the weapon of mass destruction dropped to start the Mushroom War which created the Lich King, his Well of Power and the land of Ooo.

It is unknown exactly why the Mushroom Bomb was dropped, though it was probably due to the culmination of fierce hostilities between two pre-Mushroom War peoples, or nations.

Appearance
The Mushroom Bomb has the appearance of a steretypical nuclear bomb, similar to that of the real-life 'Fat Man' bomb, detonated in WWII.

The bomb's explosion cloud is also similar in shape to real-world nuclear devices, with its distinctive mushroom shape. However this cloud is different in that it glows a deep green and is filled with many skull shaped howling souls.

Trivia

 * The Mushroom Bomb remained dormant, encased in ice and guarded by Old Marceline for well over 600 years (She and the rest of the earth were unfrozen 400 years after the bomb was frozen, which was 1,000 years ago.
 * The bomb leaves behind a crater filled with a glowing green subsence that granted farmworld Jake a increase in size, glowing green eyes, torn lips, devil horns and a lust for death like The Lich.
 * It sends off clouds of green skulls like The Lich's Well of Power. Also, the spot where Simon is buried looks like the underground subway that The Lich's Lair is in.