The cistern into which Ishmael threw Gedaliah's slain men had been made by which king?

Answer: Asa

It was a large cistern King Asa had made for fear of Baasha king of Israel; Ishmael son of Nethaniah filled it with the slain.

“Now the cistern into which Ishmael had thrown all the bodies of the men he had struck down along with Gedaliah was a large one that King Asa had made for fear of Baasha king of Israel. Ishmael son of Nethaniah filled it with the slain.”— Jeremiah 41:9 (BSB)