Shade some cells to form groups of connected shaded cells. These groups can touch each other diagonally. Cells with circles cannot be shaded. The numbers indicate the sum of the sizes of all such groups where at least one shaded cell is horizontally or vertically adjacent to the numbered cell. For cells without numbers, these sums are unknown.
(Rules and example from WPC 2019 IB)
History of the Puzzle
First appeared on Nikoli volume 138 (2012) as a suggestion from a reader by the alias of "活火山" (Katsukazan, literally "Active Volcano"). Originally named クロユニット ("Black Unit"), but later contracted to クロット ("Kurotto") on volume 141 (2013).