Eberron Player's Guide, page 111 has a list of dragonshard augments, which can be added to weapons. Only one can be added to a weapon at a time, but any player can add or remove during a short or extended rest.
The shards add bonus damage to spells/attacks that meet the criteria of the shard. i.e. (cold/lightning/radiant/vs objects/implement keyword) and most of them add damage the the roll. (1, 3 or 5 depending on level)
Others trigger on crit hits, or bloodying an opponent with it and do things like inflict necrotic vulnerability or regain hit points.
What category should these shards be created in, and how do i get it to add the damage automatically to spells including keywords "cold" or "implement"
Specifically, I'm looking to add Siberys Shard of the Mage. At least to get the damage added in correctly. Getting the interface to recognize only one augmentation per weapon is something for later.
I can say at first take that that's a pretty complicated issue. I think you would have to completely modify how weapons worked first in order to add the ability to set an augment. Then create the augments as level 0 or 99 or something items that when bought then show up in a drop down for the magic weapon.
Well, to start with, just adding the item as an available item that gives the damage boost.
You could add an item slot for "Weapon Augmentation" - that would enable you to have more than one, but only equip one at a time.
or 3 item slots:
2H Weapon Augmentation
Main Hand Weapon Augmentation
Off Hand Weapon Augmentation
but that gets complicated since items can only have one slot - you'd have to create multiple items for each item slot option.
Easier to just have one slot for Weapon Augmentation to start with.
and once it's equipped, how to script the damage to tie into the damage type or keyword of the spell. i.e. Chill Wind is an implement attack. it needs the +# damage if the shard of the mage is equipped. or if the shard that adds dmg to cold damage type, that would also need to list the additional damage.
I think whoever took this on would do better to figure out how to let weapons and implements take an augmentation. Then depending on which weapon you had equipped you would apply the augmentation effects similar to some of the other conditional scripts out there.
The big trick is the structural change to slot augs onto weapons and implements (and armor too?)
Yeah, this sounds like something similar to the wall I crashed into when trying to fix Dragonborn Rage; there's no generic way to apply a modifier to all implements and weapons that I can see (or even attacks in general; you can modify, well, everything, but then that effects skills, too).