Abridged Guide to Evil Wikia
"He had seen this place before. Green land stretching in every direction, an endless expanse of villages and fields and rivers. Men and women and children that never went far from the place of their birth, taught that they were to be content and peaceful with mother’s milk. There were no dangers, no dooms, nothing to fear in the Serenity. Only one beautiful day after another, until one day you breathed your last and the Fair King called your mortal coil to the Lands Beyond. Some would have called it a paradise, a place without sickness or war.

Anaxares the Diplomat called it a lie.
Anaxares the Diplomat[1]

Serenity is a hell or a part of the hell the Dead King conquered. The only sources of information about it as of yet are Hye Su, Malicia and Hierarch Anaxares.

It was described as fertile land dotted with villages by Anaxares. Hye Su only saw a throne hall, while Malicia described an almost utopian society while in the form of Athal. The information is relatively trustworthy as Malicia interrogated real servants in Keter and would not have wanted to blow her cover by providing wrong information.

The people of Serenity grow barley.[1]

Despite the lack of a sun, the Serenity has plenty of light.[1]

The Dead King lives in Serenity and seems to plan on becoming a God by copying Creation in Serenity. His will permeates the realm. [1]