Tattoo magic is fairly new in the middle lands. An art that traces it's origin to the eastern islands.
When a spell is transcribed to flesh the caster no longer needs an arcane device (which in the setting I'm running is required for most attack spells *) to cast that spell, as their flesh becomes the implement. The tattoo has to be exposed and glows with arcane power, the tattoo is used to focus or direct magic attacks making them no more or less stealthy to cast from.
Also spells cast from flesh benefit from +2 spellpower. But tattoo magic is not without its costs and limitations.
Mages must sacrifice 2 permanent magic points per tattoo. This is permanentl invested magic into the processes of getting the tattoo.
The number of spell tattoos a mage can have is equal to half their level rounded up. Any tattoos beyond that limit costs a sacrifice of 4 magic points permanently. These can never be recovered by leveling past the limit, it is cost at the time of getting the tattoo.
Spell tattoos can be costly. The time and material cost are big factors. Novice spells are often relatively simple, and journeyman spells are larger and more detailed. Master level spells are very large and can take up an entire limb, and are works of intricate art.
* In the setting I'm running Arane devices are required for any attack spell that says "from hand or arcane device" instead of being optional. In previous games arcane devices were optional but granted a +1 spell casting and +1 damage when used. These we're entirety setting rules.