Pope Leo VIII

Pope Leo VIII (c. 915 – 1 March 965) was a Roman prelate who claimed the Holy See from 963 until 964 in opposition to John XII and Benedict V and again from 23 June 964 to his death. Today, he is considered by the Catholic Church to have been an antipope during the first period and the legitimate pope during the second. An appointee of Holy Roman Emperor Otto I, Leo VIII's pontificate occurred after the period known as the saeculum obscurum.

Source: Wikipedia — https://en.wikipedia.org/wiki/Pope_Leo_VIII