This is a special Halloween feature only available the during week of Halloween (October 31st).
Once a living Growtopian reaches the Maw of Growganoth, he or she can use the Drop button to sacrifice items to Growganoth. Depending on the rarity of the sacrifice Growganoth can give a wide variety of prizes with an equal or lesser value of rarity.
Punching is disabled in each of the Growganoth worlds, so there is no need to worry about other Growtopians knocking you around.
Growganoth can be found in a total of 14 worlds to prevent overcrowding:
If you enter world "Growganoth14" or any other world with a number higher than 14 you will enter the world "Notworthy".
In game text:
Do not fear for your sanity.
It is already lost!
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P.S. To sacrifice items, use the DROP button when you are up by the Maw" only work in halloween