after henry's death, however, eberhard soon came into conflict with the king's son and successor otto i, who aimed at strengthening the royal authority. in 937 the franconian duke invested helmern castle near peckelsheim, located near the saxon border and garrisoned by a saxon burgrave who refused to swear fealty to any non-saxon. king otto called the feuding parties to a royal court at magdeburg, where eberhard was ordered to pay a fine and his lieutenants were sentenced to carry dead dogs in public, a particularly dishonouring punishment. infuriated, eberhard joined otto's opponents, raising a rebellion in 938 with otto's half-brother thankmar and the new duke eberhard of bavaria, the late duke arnulf's son. the revolt was soon suppressed; thankmar was assassinated at eresburg castle, and eberhard of bavaria was replaced by his uncle berthold as duke (ruled 938–945).