[AndrewBissell]

Muad'Dib fears the jihad he himself would unleash on the universe, and can't find a way to stop without ceasing his own quest for power.

The 'great enemy' that Leto tries to forestall is sometimes called Kralizec and isn't necessarily explained in detail, although there are hints it would be some kind of Ixian hunter-killer assassination machines which run out of control, and hunt down all of humanity which is within view of prescient sight. Hence, the Scattering and Leto's breeding program to create humans invisible to prescience so that humanity could never again be vulnerable to extinction from any such threat.

[Balgair]

Oh, got it! There are multiple fears then.