| The bane of wizards, these terrible spirits bring an unearthly silence to the area around them, making it impossible to recite the magical incantations involved with casting spells or reading scrolls.
“There is a silence where hath been no sound,
Silent spectres are weak, insubstantial undead that would be no real threat except for the permanent 12-tile aura of Silence they radiate. A character with even moderate weapon skills should be able to defeat them easily, but pure casters will find themselves suddenly incapable of casting spells, reading scrolls, or shouting. They can be randomly found in the Crypt, the Abyss, Tartarus, and Ziggurats, and elsewhere as part of predefined vaults. Silent spectres cannot use stairs.
Their presence is never a surprise, as their large, wall-penetrating aura can be seen long before they enter your line of sight. However, for the same reason, magic users can never use attack spells on them and would do well to buff themselves and have tactics organized before entering the aura: e.g., Shroud of Golubria, Aura of Abjuration, Ring of Flames...
Most troubling is not the spectre itself, but the possible horde that might be interposed between you and the spectre. If the horde is mighty, pick away at it, with a ready avenue of escape. Keep in mind that once you do so, the spectre itself may move closer and previously "safe," non-silenced tiles near your escape route will now be affected by silence.
A silent spectre can be a boon for competent melee fighters if there are dangerous spellcasters like liches around; that permanent aura of silence affects everything in its range, not just you.
Silent spectres were added to the game in 0.8.