Other Wichmann attestations

Contemporary Sources

Adalbert of Magdeburg, Continuatio Reginonis, trans. Simon MacLean, in History and Politics in Late Carolingian and Ottonian Europe (Manchester, 2009), 257.

955:  When he had returned from there [Bavaria] the king sent his army against the Slavs, where he won a similar victory and struck them down with a great massacre.  Wichmann was expelled.

Necrology of Lüneburg (Accessible here) 

September 22:  Count Wichmann and many others slain, and Hoico slain.

Later Sources

Thietmar of Merseburg, Chronicon, trans. David Warner, Ottonian Germany (Manchester, 2001), 95, 99, and 101 respectively.

II.6:  After being reviled by his uncle Henry, he [Liudolf] returned to the city and to his rebellion, having won over Count Ekbert and many of his uncle's warriors... As his father pursued him, Liudolf tried to regain the city of Mainz by bribing Duke Dietrich and Count Wichman who were continuing the siege. Of the two, Dietrich stood firm, but Wichmann was corrupted by his enticing promises.

II.12:  As these events were transpiring, the Slavs started a horrible war at the instigation of Counts Wichman and Ekbert under the leadership of Nacco and his brother Stoigniew. Lacking confidence in his own ability to defeat them, the commander, Herman, asked the king for help. Energetic as he was, the latter took a strong force and invaded those northern regions which, as scripture teaches, so often produce evil. There, the king had Stoigniew beheaded, after capturing him in a wood in which he had hidden as his supporters fled. He pursued the authors of this outrage, the brothers Wichman and Ekbert, sons of his maternal aunt.

II.13:  Also, Wichmann was conquered.

Sigibert of Gembloux, Chronica sive chronographia universalis (Accessible here)

958:  Wichmann, crossing the Elbe rebellious from the figure of King Otto, led Slavs into his kingdom, who the king strongly conquered, with a subregulus slain with many of his men, and 700 captives decaptitated.

967:  Wichmann, rebel against Emperor Otto for a long time, is killed by Mieszko, regulus of the Slavs, friend and soldier of the emperor.

Annalista Saxo (available here) incorporates much of Widukind, including the Wichmann narrative, verbatim.