Runes

An ISPD officer with a magical symbol burnt deep into his left palm. It's a scroll, maybe, or a 1/day, or a Glyph of Warding that's got a trigger of "touch someone" and triggers a... nasty spell. Like Scar from FMA.

Classifications: a glyph is a one-time, on an object. A symbol  is persistent, and on an object. A mark is a one-time on a person. A sigil is a persistent, on a person. All of these are runes, as a whole.

With Glyph of Warding you can basically make a rune of whatever. Powering them, per day, just means being able to expend a spell slot each day. Or for each use, basically. It's almost like a new spell known. For casters that have spell slots, cool. For the characters with no spell slots to burn, how about getting a one per day for free with a greater rune cast thing. Or else, I dunno, use the vitality casting rules. So if blood is magic, and you have a rune on you, you can fuel it with hp. Treat them as sorcery points, but maybe double cost. So activating your 3rd level spell is a 10 hp endeavor, for example. Or combine the two: you must spend hp to do it, but max out at 1/day. Or else you take more damage each time. 10 hp the first time, 20 the next, 30 the next, etc. This prevents a barbarian from getting a glyph of fireball and just going to fucking town (trading 10 hp for a 5d6 fireball every turn, damn).

Popular concealed 'weapon': inflict wounds on a mark or a sigil, triggered on touch or triggered with a command word. Have a high-enough level caster put it on there and you have a very deadly one-time-use spell activation in the hands of a front-line fighter. You can do the same with, for example, an eggshell. Glyph of warding as a 3rd level spell, trigger "when the eggshell is broken," target "any living creature in contact with the eggshell," effect is a 3rd level inflict wounds doing 5d10 with no save. Fill the eggshell with black glitter for fun and weight, and bam, you have the deadliest rogue in history. The cost would be minimal, too: a spell scroll for a 3rd level is an uncommon magic item, and the cost for creation is 500 gp. 500 gp for a 5d10 no-save is brutal.