With millions of HP and nigh immune to everything but pure magic, it summons waves of monsters each considered world-ending calamity by itself. It will only stop summoning when silenced, but even strongest silence spells only last a moment, if they succeed at all. Even if you manage to whittle it down, it will start regenerating millions of HP: most likely multiple times the damage you can put out. Yet when you see it, it's too late: it will simply teleport you back near itself if you try to run.
Beware the crab.