A locus is the object a spellcaster channels magic through: a wand, a staff, an orb, an instrument. It is also a weapon, a third kind beside melee and ranged weapons, that strikes with magic instead of steel.
What a Locus Is
A locus (plural loci) is a crafted object that holds and focuses magic. Wands, staves, grimoires, orbs, censers, astrolabes, mirrors, chalices, lanterns and musical instruments can all be loci. Your class says which loci you are trained with.
A locus is magical, so you must invest it to use it. It counts as one item toward your investiture limit.
Locus Strikes
You can attack with a locus you are holding by making a locus Strike , which works like a Strike with a weapon, with these differences.
Attack roll. You use your spell attack modifier instead of a weapon's attack modifier, and the attack roll is made against the target's Deflection.
Damage. A locus deals d4 damage. The number of dice follows your proficiency with loci, just as it does for weapons, as shown in Weapon Damage Dice, up to five dice. You don't add an attribute to locus damage.
Damage type. Each locus deals its own type of damage, named in its entry. A musical instrument deals sonic damage.
Multiple attack penalty. Locus Strikes count toward your multiple attack penalty, and it counts toward theirs.
Quality and Runes
A locus has a quality like a weapon, which lets it hold up to three runes.
Locus Traits
A locus can carry weapon traits such as agile, reach, deadly or parry. These traits only change locus Strikes: they never change the spells you cast through the locus.
Staves
A staff is at once a locus, a melee weapon and a magical store of spells. It holds charges equal to the highest rank of spell slot you have when you prepare for the day, and you spend those charges to cast the spells written in it.
Instruments
A bard's instrument is a locus: they fight with it, cast through it and carry their compositions in it. The Bard describes the instrument loci a bard can use, including the coda, an instrument that stores spells as a staff does.
Still Being Written
The range of a locus Strike, the full list of loci with their damage types, and locus critical specialization effects are still being decided. They will be added here.