The light cast by a handful of world-ending spells. He’s one of the oldest living beings among the Mythos. The... describes it thus:
Above them all swirled the storm. Doom-light, I called it – for there was no sunlight, no sky, only that uncomprehending whirl, gray and flashing purple, thick like smoke and moving like water. All could be seen – but it was not natural light, not from sun or moon or stars or flame, and it turned all it touched pallid.
It’s extremely rare, and not many have seen it. He’s one of the oldest living beings among the Mythos. The... has stated more than once that each time it’s been seen, the fact that the world hasn’t ended was enough to make him believe in miracles, or at least in power beyond what’s seen.