The Tias: Soulbound Gems As Magical Keepsakes

tldr; there are dozens of magical gemstones, each with a soul-bound princess trapped inside of them. Each conveys a small bonus, while collecting several conveys a cumulative set bonus.

The Ishnindah is not one of history's actors. She lurks in the background, plying her trade with the stoic fools of each era. Her footprints stretch across millennia, backwards and forwards.

Though she has lately preferred body parts as payment, there was a time when she preferred maidens of royal extraction. Want a spell that causes an enemy to only gain nutrition from cannibalism? Turn over a princess. The Internal Struggles of three centuries ago were largely an arms race of purchasing death-hexes from the Ishnindah. The royal families of the great cities mined their extended family trees for suitable candidates, turning them over by the dozen. Much magical lore was accrued from these transactions (the Ishnindah's well of magic does not run dry), though it was largely destroyed later, when the faith of the Idolatrous Lords waxed supreme.

The Witching Ways
The Ishnindah confounds analysis. But we know very well what happened to the princesses given over to her: their soul were trapped in gemstones, used to power their magics. There are at least fifty of them, each made by the Ishnindah and traded away. Over the centuries they have fallen into the hands of many adventurers and monster troves. In my game there is a flat 5% chance per incidence of treasure that a tia is among them.

The gemstones are called "tias". Keeping them in your possession only counts as one magical item, total. I use a trinket slot. If you use the absurdly granular Pathfinder 17+ item slot system, please stop. If you persist, maybe use the amulet slot.

The Tia Stones

Each tia stone conveys a small magical bonus, while the total number of tias equipped by a character conveys a cumulative "set bonus".

Set Bonuses (total mumber of tias: bonus)

• 3: You have resistance (half damage) against acid and poison.
• 5: You have +1 to all of your saving throws.
• 10: Your spells inflict +1 damage.
• 20: +2 to all saving throws.
• 35: Your spells inflict +2 damage. 
• 50: You cease aging, gain +3 to all saving throws, and can infallibly detect lies and detect magic as per those spells.

Individual Stones

I really dug deep for these t-named gemstones. You should come up with your own, too.

• Tia Topaz: You heal 1d6 hit points from healing spells of 1st-level or higher.
• Tia Tigerseye: You have a +4 bonus to Sense Motive checks.
• Tia Turitella: You are immune to slow and similar effects.
• Tia Tanzanite: You have resistance (half damage) against electricity.
• Tia Tourmaline: You can cast cure light wounds twice per day.
• Tia Titanite: You can cast a weak version of dimension door, once per day. It functions as the normal spell, but with a range of only 100 feet.
• Tia Tektite: You ignore the first incidence of ability score damage, each day. If that doesn't match your rules set, it instead makes the player immune to the first debuff to affect them, each day.
• Tia Tortoiseshell: You have +1 AC.
• Tia Turquoise: Outsiders and spirits have disadvantage when attacking you.
• Tia Tsavorite: You have resistance (half damage) against attacks of opportunity.
• Tia Thomsonite: You have +3 initiative.
• Tia Tugtupite: You are immune to bleeding effects. You have +5 max hp.
• Tia Taaffeite: You can cast meld into stone once per day.
• Tia Tinaksite: You are immune to damage and suffocation from non-magical heat and fire. For the purposes of this tia, magma and other geothermic heat is magical.
• Tia Tyuyamunite: You shed a ghostly green light within a short radius. You are immune to blindness, magical or otherwise. If, for example, your eyes were gouged out, you could still see so long as you had this tia.
• Tia Tsumebite: You can see through barriers of 1 inch thickness or less, including fog and many doors. You can't see through lead or other unusually dense materials, though.
• Tia Tsumcorite: You are immune to color spray, prismatic spray, and any other "color-based" spell. You have +2 speed.
• Tia Trona: Your fire spells inflict +1d6 damage.
• Tia Triphylite: You cannot be knocked prone.
• Tia Tridymite: You do not suffer any ill effects from excessive atmospheric pressure or the lack thereof. You are immune to decompression sickness. You can also hold your breath for up to a week.
• Tia Tremolite: You have fire resistance (half damage).