Scalelord Reckoner's triggered ability resolves before the spell or ability that caused it to trigger. The ability will resolve even if that spell or ability is countered.

If the only nonland permanents on the battlefield are Scalelord Reckoners and Dragons controlled by players who control Scalelord Reckoners, any spell or ability that targets one of those Dragons will cause an infinite loop of triggered abilities and the game will immediately end in a draw.