Apollyon a name for the Devil, in Revelation 9:11.

In Bunyan's Pilgrim's Progress, Christian has to fight to get past the ‘foul fiend’ Apollyon, who blocks his path.

Recorded from late Middle English (in Wyclif's translation of the Bible) the name comes (via late Latin, in the Vulgate) from Greek Apolluōn (translating Abaddon), and ultimately from apo- ‘quite’ + ollunai ‘destroy’.

Apollyon

The destroying angel or prince of the underworld (Rev. 9:11), synonymous with Abaddon.

Sources:

Barrett, Francis. The Magus. 1801. Reprint, New Hyde Park, N.Y.: University Books, 1967.