It loves the crying of children.
About Duskull, #355

Japanese Name: Yomawaru (ヨマワル)
Species: Requiem Pokémon
Type: Ghost
Dex Numbers: #355, #153, #189
Height: 2'07" / 0.8 m
Weight: 33.1 lbs. / 15.0 kg
Egg Group: Amorphous
Pokédex Color: Black
Body Style:
HP: 20
Attack: 40
Defense: 90
Sp. Attack: 30
Sp. Def: 90
Speed: 25
Total: 295

Growth Rate: Fast

First introduced in Generation III, Duskull is a Ghost-type Pokémon and is classified as a Requiem Pokémon. It is #355 in the National Pokédex. It evolves into Dusclops starting at level 37, which can then evolve into Dusknoir when traded while holding a Reaper Cloth.

The shiny version of Duskull appears to have a red cloak opposed to its normal black or dark gray colored cloak.

Pokédex Entries
Duskull across different games

Ruby, Omega Ruby

Duskull can pass through any wall no matter how thick it may be. Once this Pokémon chooses a target, it will doggedly pursue the intended victim until the break of dawn.
Sapphire, Alpha Sapphire

Duskull wanders lost among the deep darkness of midnight. There is an oft-told admonishment given to misbehaving children that this Pokémon will spirit away bad children who earn scoldings from their mothers.
Emerald

A glare from its single scarlet eye makes even burly grown-ups freeze in utter fear. It is a nocturnal Pokémon that roams about under the cloak of darkness.
FireRed, LeafGreen

Making itself invisible, it silently sneaks up to prey. It has the ability to slip through thick walls.
Diamond, Pearl, X

It doggedly pursues its prey wherever it goes. However, the chase is abandoned at sunrise.
Platinum, Black, White, Black 2, White 2

It loves the crying of children. It startles bad kids by passing through walls and making them cry.
HeartGold, SoulSilver, Y

If it finds bad children who won't listen to their parents, it will spirit them away—or so it's said.

Etymology
On names and meanings

Its English name, Duskull is a combination of the words "dusk" and "skull." Its name in other languages more or less use the same imagery, though some of them also reference dark mythological creatures or draw upon the concept of nightmares. These themes are explored further down the page. (Soon!)

The Spanish and Italian versions of the game also use "Duskull."