Tales of Viridia
Every few centuries, a great horde of Orcs with a mighty warlord at its head sweeps out of the Plains of Mauhulakh and across the land, conquering and pillaging as they go. Inevitably, the warlord dies and the horde is dispersed and driven back. This dispersal seeds the affected land with a fresh resurgence of Orcs, some of whom breed with Humans, creating Half-Orcs.
Half-Orcs are tolerated all but the most conservative Human and Orcish societies, although many mistrust them. They inherit the skin tones and blocky features of their Orcish kind, and sometimes tusks and curled horns. They tend towards being taller and more muscular than the average human, but not quite to the same extent as an Orc.