Whenever Dream Fighter blocks or becomes blocked by a creature, Dream Fighter and that creature phase out. (While they're phased out, they're treated as though they don't exist. Each one phases in before its controller untaps during their next untap step.)
10/4/2004 Dream Fighter affects all creatures that end up blocking it, including ones that are members of bands.
10/4/2004 If Dream Fighter is assigned to block a creature with Flanking, the Flanking ability and the Dream Fighter ability both trigger. The active player’s Flanking effect is put on the stack first and resolves after the Dream Fighter’s phasing out effect happen, so the Flanking does nothing.