My version of Mortarion shows him while attacking one of the beautiful worlds of Ultramar - specifically a beautiful and lush garden. He has just arrived but his influence already starts to spread corruption and decay. The touch of his cloak will let the giant blossom under him rot away in seconds and we see the beginning of that in the miniature.
The base and it's beauty is one of the major contrasts against the old evil and decay that Mortarion stands for.