Like The Two Towers a year earlier, when The Return of the King was the beginning of a new format allowing only cards from that set. Cards from Siege of Gondor and Mount Doom were also allowed once those sets were released, but for the first time a set was released between those two which wasn't part of any block at all -- Reflections. In sets 7 and 8 only Sneaking
could cancel skirmishes, a stark departure from previous blocks. Slunk Out of Sight (10U115)
opened the door back up for Hobbits in set 10, although a rules change accompanied set 9 which made cancelling Ring-bearer skirmishes illegal in all formats even though the set itself is excluded from King Block. Unlike Fellowship Block and Towers Block, King Block has not enjoyed the same level of community enthusiasm over the years. Because it does not include Reflections and most of its strategies are either completely contained within the block or blatantly missing cards from earlier sets, there is often little draw to it over its standard counterpart: Movie Block.
Decklists for King Block can be found at this subforum of the Last Homely House.
Restricted list
After King Block was retired and Shadows was released, Decipher also revamped their Open format. Rather than cards being either banned or permitted, a new R-list restricted certain cards (loop-enablers or cards deemed too far above the power curve) to only one copy. This same R-list was enforced for Block tournaments as well, despite the interactions leading to the restriction only occurring from cards between blocks. Steadfast Champion (7U49)
is on the list thanks to Treebeard, Keeper of the Watchwood (10R18)
-- a card which, while from the block, can't be played in it (the only other 
Gandalf companion is Gandalf). Therefore there are technically two "versions" of King block, the original one without an R-list and the updated tournament one with an R-list.
