Given the words 'Mansion' and 'Swamp', I came up with the idea of an ancient wizard's house, built deep within a possessed swamp. The house has been passed down through centuries of the same wizarding bloodline, as the house is enchanted to stop the swamp from expanding and destroying the nearby villages. When the grandson of a long-lost relative is called to look after the house, he mistakenly lets a demon loose into the swamp, killing the only surviving wizard in the family.
The main tower is filled with portraits of wizards passed, who have used their abilities to protect the house. The architecture is also obscure as every wizard has expanded the house and given it part of their personality. The crucifix on top of the house is connected to the study at the very peak, where magic is performed to keep the swamp from expanding. The swamp tries to grow over the natural light that the crucifix needs to work, which is why the magical element is so essential.
I contemplated an actual figure, emerging from the swamp to act as the antagonist, but I found the idea of an evil, living swamp to be much more interesting (particularly for animation).