The mechanic
The Moor Hag opens the fight with 4 dormant armored mounds already seeded along the path ahead of her, at fixed points roughly one-fifth, two-fifths, three-fifths, and four-fifths of the way to your base. The mounds are stationary and immune to crowd control, and they do nothing while they wait.
The mechanic is a race. Each mound the hag actually reaches rises 4 seconds later as a drowned thrall that marches ahead of her toward your base. But a mound you destroy before she arrives raises nothing at all, so every mound you crack in time is one thrall that never spawns.
When the hag herself dies she ruptures into 5 fast bog mites that scatter for your base as a final cleanup task.
How teams beat it
Race the hag to her mounds: focus fire the mounds down before she walks into them, and each one you break is a thrall you never have to fight. Armor pierce helps crack the mounds quickly, and keep area damage in reserve for the bog mites at the end.
Like every boss, it fights inside the showdown rules: kill it and it respawns about twenty percent stronger on the rival's lane, let it leak and it crosses unescalated. The full showdown rules are in How a match works.
