A hexagon level (or skull level) is a feature which marks a level as a "Hard Level" in game. Notice that, the "hard" label may not be the same as the difficulty rated by this wiki. They are shown as skulls as of now.
They were introduced in version 1.60.0 in mobile (HTML5), and at the release of Hoax Hollow in Flash. On 2016, they disappeared numerous times, but they were restored.
There is no special bonus for clearing a hexagon level in most situations. However, exceptions include filling the Piggy Bank, getting more points for Fantastic Five, or getting "scrolls" for Candy Chronicles.
A level may change from normal to hexagon or vice versa. This generally happens when a level is redesigned, but sometimes, it happens without redesigning. All hexagons may be removed or added.
Hexagons do not necessarily reflect the difficulty of a level in the wiki, as King rates it.
It is possible that King will track the number and ratios of successes and failures relative to other levels. Some early levels with a higher degree of challenge may also be determined as hexagonal.
Before, the set of hexagon levels in Flash may not be the same as the set of skull levels in mobile excluding recently released web-only levels (for example Level A is a hexagon only in flash, but exists as a non-hexagon in HTML5). Hexagon levels in Flash were usually updated prior to HTML5.
Around February 2017 in mobile (HTML5), thunderstorm levels are implemented. They mark levels as "super hard levels." They are represented as skulls, but, in addition to that, the level button is now dark blue hiding the level type, and a dark blue thundercloud hovers just over it. In 28 June, they were released in Facebook as part of the HTML5 rebrand.
Later, in a mobile update in September 2018, owl levels are implemented. They mark levels as "nightmarishly hard levels". They are represented by skulls and the level button is dark blue hiding the level type, and Odus appears in the intro.