Adamastor is a Portuguese Greek god. He was invented by the well-traveled poet Luís de Camões in 1572, and has been going strong ever since. Camões wrote fantastical accounts of sea voyages, and his epic poem Os Lusíadas (The Lusiads) is often considered to be Portugal's national epic. In the fifth canto he introduces a giant, monster-like sea god.
I was weighed down with fear,
And our hearts were filled with dread;
The black sea roaring a continuous roar
As if it were throwing itself against a cliff.
'Oh divine power', I said, exalted,
'What threat or what secret
This clime and this sea to us presents
That seems a bigger thing than a storm?'
Even as I spoke, an immense shape
Materialized in the air, robust and true,
Deformed and of enormous stature,
Its face laden, with a squalid beard,
Its eyes sunken, its appearance frightful,
Its complexion earthy and pale,
Covered in dirt, with curly hair,
Its mouth black, and its teeth yellowed.
So large were the limbs that I can
swear to you, that it was the second
only to the strange Colossus of Rhodes,
One of the seven miracles of the world:
It spoke in a voice thick and horrible,
That seemed to come from the depths of the sea:
It chilled us in our flesh and made our hair stand on end
We could see and hear only this thing.
Adamastor plays a big part in the poem, being the focus of strophes 41 through 60. He is described as a monstrous sea god, the guardian of the entire coast of Africa, and of the Cape of Good Hope in specific. He relates to the sailors that he is one of the Gigantes of Greek mythology, who tried to fight the Olympians but lost, and who were then all killed or banished. He explains that his life is pure misery, and having done so at great length, he retreats into the depths.
It is a very nice poem, not as stuffy as you would expect from a poem over 400 years old. (I will be the first to admit that it does not translate as well into English as one might hope). It has been a centerpiece of lusophone literature, and Adamastor has become a reoccurring character in works by a wide range of authors.
Any number of works (poetry in particular) mention him, nearly always in the context of sailing around the Cape of Good Hope. He has also appeared in works in various other languages. He appears in Victor Hugo's Les Miserables ("If Adamastor were to appear to him, the street Arab would say: 'Hi there! The bugaboo!'"), in Giacomo Meyerbeer's French opera L'africaine, and he also appears multiple times in the works of Alexandre Dumas. He is still in circulation; in 1993 he made an appearance in the South African author Andre Philippus Brink's Cape of Storms: The First Life of Adamastor.
Adamastor is particularly well-known in Portugal, and in fact there is a well-known esplanade in Lisbon named after him, overlooking the Tagus river. It contains the Miradouro de Santa Catarina (the Santa Catarina overlook), a garden which gives an excellent view of the river, the April Bridge, and the Cristo Rei monument. This garden includes a statue of Adamastor. The statue is not actually very impressive, but the view is, and it is a popular gathering point.
If you would like to read Os Lusíadas in its original Portuguese, http://www.oslusiadas.com/ is an excellent site. I have not been able to find an English translation (hence my own rather unpoetical translation given above), and the few fragments I have found translated are not really worth the trouble of reading.