Mushroom Bomb

The Mushroom Bomb first appears in Finn the Human (episode) and is the weapon of mass destruction dropped at the end of 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.

Apperance
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 to real-world nuclear devices, with its distinctive mushroom shape. However the cloud is different in that it glows a deep green and is filled with many skull-shaped faces.

The Mushroom Bomb is almost only seen encased in a layer of ice.

Trivia
The Mushroom Bomb remained dormant, encased in ice and guarded by Old Marceline for well over 400 years.