Enter numbers from 0 to 9 into the grid, one into each region. Regions with equal numbers cannot share an edge (they may touch diagonally). For each grid point where four regions meet, the sum of the numbers in those regions must equal 10. For your convenience, all these grid points are marked with a small black dot.
(Rules and example from WPC 2019 IB)
History of the puzzle
Invented by Jürgen Blume-Nienhaus (Germany) in 2019. The name Terra X probably comes from a long-running German documentary show of the same name.