Sorcerers and Spelunkers
Demogorgon is an abyssal lord of incredible power.
During their quest for a safe haven, the party was led to the Qua Toa by their resident fishy fish, Shusshar. However, upon arrival, they were attacked by several Qua Toa claiming to believe in a diety Shusshar had not heard of before.
Many misguided people of Sloobludop pledged worship to a being they called the Deep Father. The daughter of the chieftan sacrificed her own father by flame to bring forth this Deep Father: Demogorgon. The demon prince appeared and immediately razed the city.
The party fled. When they later passed over the dark lake, no life existed.