The Nariai solution is the limit of the largest black hole in a space which is de Sitter at large distances, it has two horizons, the cosmological de Sitter horizon and a Schwarzschild black hole horizon. For small mass black holes, the two are very different--- there is a singularity at the center of the black hole, and there is no singularity past the cosmological horizon. But the Nariai limit considers making the black hole bigger and bigger, until its event horizon has the same area as the cosmological de Sitter horizon. At this point, the space-time becomes regular, the black hole singularity runs off to infinity, and the two horizons are related by a space-time symmetry.
