Putting Ahimaaz into battle is considered playing him and (assuming you're using the teal one) would trigger Kidron Valley and you could choose to discard Plagued with Diseases, however KV doesn't negate PwD so the decrease on Ahimaaz would remain and he would be discarded by game rule. Assuming you had access to a Lost Soul with Ahimaaz, Chariot of Fire would then trigger and shuffle Ahimaaz into your deck.
Plagued with Diseases
Decrease all opponents’ Heroes by 0/2 (or 0/3 if you have the fewest Redeemed Souls).
Ahimaaz
Protect this Hero from capture. You may look at one opponent’s hand or cards face down in a Site. You may then withdraw Hero from battle unharmed.
Kidron Valley
When you put a teal priest in play, you may discard a Curse, an Artifact with a capture ability or an Artifact depicting an idol.
Chariot of Fire
Following your rescue attempt, return all Heroes in your discard pile to your draw pile. Shuffle draw pile.
