What is the technique in "A vast silence reigned over the land."?

Personification, if you're talking about figurative language.

Explanation:

Personification is a literary device that gives human-like qualities to something like an inanimate object, idea, or substance that doesn't have one in real life. In this case, silence, the subject of the sentence, cannot physically rule over the land. It can only do so figuratively and with a different meaning. In this phrase, silence "reigned" over the land means that the environment being described was very quiet.

a source card is mage during research to tell the facts about the source one is using.

from the excerpt from the "the dark game" the reader can infer that folke cronholm was spying for the allied nations.