This is the 12019th most frequent Russian word.
зеркалом
"Зеркалом" means "mirror" in instrumental case in Russian.
The word 'зеркало' refers to a mirror and is used in the instrumental case here, 'зеркалом', which indicates the indirect object of the action.
Я смотрюсь в зеркало каждое утро.
I look at myself in the mirror every morning.
Here, 'зеркалом' is in the instrumental case, showing the means (the mirror) by which the wall was decorated.
Он украшал стену большим зеркалом.
He decorated the wall with a large mirror.
In this example, 'зеркалом' is in the instrumental case to indicate the tool (the mirror) used to accomplish the action of illumination.