In dNethack, Archon denotes a whole group of related monsters; the most common type of lawful angel. The vanilla Archon is known as a Throne Archon in dNethack. As with most other angelic monsters, Archons will only be generated in the Planes.
Trumpet Archons are summoners, and should therefore be a high-priority target.
The Light Archons' volleys of silver arrows make them particularly dangerous to Vampire characters.