Well…her name is actually “King of Shadows” but the TCG mixed up “Darkness” with “Shadows” on a different monster…
Kageoh* Duke Shade / Duke Shade, King of the Umbra*
Dark Fiend / Effect
You can only use this card name’s (1) and (2) effects once per turn each. You cannot Special Summon monsters during the turn you activate any of this card’s effects, except DARK monsters.
(1) You can Tribute any number of DARK monsters; Special Summon this card from your hand, and if you do, this card gains 500 ATK for each Tributed monster.
(2) If this card is Normal/Special Summoned: You can target 1 Level 5 or higher DARK monster in your GY; add it to your hand.
*) Kageoh here is literally “King of Shadows” but “Prometheus, King of Shadows” already exists in the TCG, so we’re exercising some creative liberty here.
Source is the official Yu-gi-oh! OCG Twitter page