In 839, according to the Chronica S. Benedicti Casinensis Landulf took the initiative in freeing Siconulf, the imprisoned brother of the assassinated Prince of Benevento, Sicard.
Siconulf was proclaimed Prince in Salerno and Landulf pledged his city to him.
He had fought for Sicard against Naples in his early years, but he concluded a peace treaty with the Neapolitans in order to be able to fully enter the war against Radelchis.
Radelchis called in Saracen mercenaries and they sacked Capua in 841.
It seems that, by the end of his life, he was employing the title of Count.