leucoderma, piebald skin
癜 centers on a skin condition involving irregular white patches: leucoderma or piebald skin, where pigment is lost in spots.
癜 combines 疒 (sickness) with 殿, which likely contributes the sound and may relate to a spreading or patchy quality, but the exact historical development is uncertain.
The sickness enclosure 疒 wraps around 殿, a shape suggesting a grand hall. Picture a grand hall with patchy white spots on its walls, like leucoderma on skin.
For デン, imagine a den where someone with piebald skin rests: den -> デン, and the sickness enclosure shows it's a skin condition.