Translated from the back of the book:
"'Gate of Terror'
Maraskan. Steaming Jungles, steep crags, poisonous animals, rebels, assassins and old reptilian ruins - not a land that any one blessed by the goddess Hesinde with an ounce of reason would enter. But this should not scare the heroes, for a mission from highest clerical orders leads them to this damned island, so examine a spectacular raid - with booty that might open the gate of terror...
'Gate of Terror' is the third part of the series 'The Marks of the Seven', a campaing co-authored by the most popular authors of DSA."
Additional user summary (contains spoilers):
Part 3 of 7 of the campain "The Marks of the Seven" about the return and downfall of legendary black wizard and demon master Borbarad.
In part three, the heroes' investigations on the jungle island Maraskan uncover a greater demon tunneling for magical ore, which is then used to open a gate to the realms of the arch demons who are the corruptors of the six elements of nature.