Complex sentence: The site was likely used for cultic congregations since antiquity, centered on a stone now in the crypt of the church, in medieval tradition taken to be the place of execution of Saints Felix and Regula.
Split into simpler sentences: The site was likely used for cultic congregations since antiquity, centered on a stone now located in the crypt of the church. In medieval tradition the site is taken to be the place of execution of Saints Felix and Regula.