[3.5] definition of threaten

[ojayaba]

my question is this, Mage slayer says that any spell caster you threaten automatically fails its concentration check to cast defensively. Does this mean that even friendly mages cant cast beside you?

[Morcleon]

my question is this, Mage slayer says that any spell caster you threaten automatically fails its concentration check to cast defensively. Does this mean that even friendly mages cant cast beside you?

By RAW, they only fail if they try to cast defensively.

By RAI, I'd rule it as "you don't 'threaten' an ally".

[Deophaun]

Technically, you threaten squares, not creatures or objects. Enemies (and RAW does specify "enemies") that take certain actions within squares you threaten then provoke AoOs.

So yes, your allies can safely cast defensively while adjacent to you. Whether mage slayer actually works by RAW is another question.

[Biffoniacus_Furiou]

From Mage Slayer:
Spellcasters you threaten may not cast defensively (they
automatically fail their Concentration checks to do so), but
they are aware that they cannot cast defensively while being
threatened by a character with this feat.

From the PHB Glossary:
threaten: To be able to attack in melee without moving from
your current space. A creature typically threatens all squares within
its natural reach, even when it is not its turn to take an action. For
Medium or Small creature this usually includes all squares adjacent
to its space. Larger creatures threaten more squares, while smaller
creatures may not threaten any squares except their own.
threatened square: A square within an opponent’s reach. Generally,
characters threaten all adjacent squares, though reach
weapons can alter this range. Certain actions provoke attack of
opportunity when taken within a threatened square.

So presuming it means spellcasters [i]you threaten, the feat works. Regarding threatened squares, you automatically threaten every square within your reach regardless of its occupant, though a given creature's square is only threatened if it's within an opponent’s reach. The wording of Mage Slayer is whether you threaten them (i.e. their square), which is automatically every square in reach, and not whether their square is threatened by you, which would only count if it's within an opponent's reach. Just let your DM decide, but remember you don't always have to cast defensively, there's not even any reason to if no opponents threaten you. You can also just 5-ft. step before casting to either get out of opponents' reach or get out of the Mage Slayer's reach.