"The king had found peace."
Translation:O rei tinha encontrado a paz.
Using the article brings the same behavior of other nouns that seem to be greater entities, such as "life", "nature", "time" and others.
There are sentences that use those in English without articles, as if they were proper names. (Nature is our mother, time will tell us the truth, life is great....). In Portuguese, these should use the article. These things are "the things", great, important, they are not "adjective-like".
"Paz" can fit one of those "big things", one that everyone aims at.
But it also fits a more regular use, as if it were just an amount of something, like in "He found some peace".