Alright more Downy explanation, he's using a summoning ritual and bringing it to overload. The ritual when casted (Which he's already done.) will continue to summon monsters at a rapid pace from the 3 conduit circles. They will continue such action until the mana in the area is completely depleted (About a day's time.) after such they will then begin a meltdown phase which creates an explosion similar to a nuclear bomb, complete with mana sucking and mana fallout that leaves nothing left in it's place.
The cause for such a devastating spell is quite simple, as Downy just wants to return to his own world yet he can't do that alone, he needs to create what is essentially a beacon loud enough for his true summoner to find him and then they can get back together.
Also to destroy the spell the 3 conduits must be destroyed first, destroying the main energy portal first (The one Downy's at.) will just cause the explosion anyway taking Downy and anybody in a large radius with it. (I'd say on a lever far greater to the bombs dropped in Japan.)