"Draw the enemy to him." is not a horrible plan even if he doesn't outright have greensight. a. as far as he knew, the dragons were always there. he might not have even known dragons had gone missing. Tricking this current generation of men into bringing him a dragon, could have been the plan all along. b. if they don't bring him a dragon, they build an unassailable army/fortress and park it right there. and build more and more strength in an unending winter. Living men, can not wait out the immortal undead. so, they must deal with the problem now, and not just ignore it. so, back to option A. The only flaw here would be, would the winter eventually end? if so, yes, theres a flaw in the plan. I doubt that is the case though. it would make sense if he was creating a perpetual winter.