If any PCs refuse, Elminster will shrug, say, "Be nothing, then," in tones of cold scorn, and turn them invisible by means of a spell of his own – invisibility that does not wear off for 3-12 days, and cannot be removed by other magic short of a limited wish, except by Elminster himself.
If PCs press Elminster for informal tion [sic] as to what they should do, and why, he will hurry on toward the temple, smiling, and say, "All in good time, all in good time…save the world first; ask questions later!"
If any PC should attack Elminster, or try to sneak away, they will find themselves levitated sixty feet straight up in the air, helpless to move or undo this magic (despite any spells they might try).