A'rklonn uses what he calls a 'unified magic focus' since he's trained in several magical disciplines. he isn't that great a conjurer, and has very little healing knowledge; most of his knowledge in it is elemental/offensive, but that's sort of off topic. He has a staff he crafted from proper thaumaturgical components, but it's hollowed out in the central 'staff' area. Inside he has a wooden focus that allows him to use his Conjury, and has carved the inside and outside of the staff during construction all over with geometries inked in. The crystal also forms the geometries, cycling through them using glamour imaging methods, similar in a way to Freed Justine's 'Jutsushiki' from Fairy Tail, which is very, very similar to Arcanima in a lot of ways with the inks made into the forging itself as a partial catalyst.
The tradeoff to this is that a universal focus is a: expensive to make, b: very time consuming to construct, and c: as it acts as a focus for multiple types of magic, more stress is put on it. So he keeps other foci on him in a pinch, like a ceremonial dagger, a small cliffnote arcanima book for basic spells (diary sized), and a simple unworked wooden focus for his conjury if his unified focus happens to crack or break under the stress. The smaller foci will naturally not produce magic as destructive or effective, but can generally handle normal targets long enough for the problem to be solved.
As far as inked in tattoos, it is entirely possible. However, a better alternative for those that want to go this method, is the Riddler method. That is, a robe with all your inking done on the inside of your clothing. Transcribe your better spells on the cloth with special inks, inside the outfit. That way it looks like you're disarmed, or are not carrying a focus, but in reality your clothing actually is the focus. You might only be able to put a limited number of spells on such a thing, but in a pinch, it could save your life if your book burns or is damaged. The key to using magic in battle is resource and battlefield control. Mages are smart; a good mage always has a backup. Just like a good fighter always has a dagger in his boot or something hidden in his clothing in case he loses his sword or axe, a good mage will have a smaller focus or alternative focus item on his person in case his primary one is removed form his possession.
In the instances I just choose to say a smaller focus can only handle a certain number of uses, or maybe it can only use certain spells. But that's up to the roleplayers in question, so it's situational. Go with what works for the group, more or less. But don't sell your mage short. No caster is going to be dumb enough to not carry backup foci. Also, if we pay attention to the MSQ (slight spoiler here), Y'shtola casts without holding on to her focus (It's still on her belt), meaning you only technically need to have it on your person, but that's likely only if you're trained enough. I'm guessing that 'holding' the focus is a method that makes the casting easier, but when you're stronger, you just need it there in contact with you in one way or another. That's my headcanon anyway.
The tradeoff to this is that a universal focus is a: expensive to make, b: very time consuming to construct, and c: as it acts as a focus for multiple types of magic, more stress is put on it. So he keeps other foci on him in a pinch, like a ceremonial dagger, a small cliffnote arcanima book for basic spells (diary sized), and a simple unworked wooden focus for his conjury if his unified focus happens to crack or break under the stress. The smaller foci will naturally not produce magic as destructive or effective, but can generally handle normal targets long enough for the problem to be solved.
As far as inked in tattoos, it is entirely possible. However, a better alternative for those that want to go this method, is the Riddler method. That is, a robe with all your inking done on the inside of your clothing. Transcribe your better spells on the cloth with special inks, inside the outfit. That way it looks like you're disarmed, or are not carrying a focus, but in reality your clothing actually is the focus. You might only be able to put a limited number of spells on such a thing, but in a pinch, it could save your life if your book burns or is damaged. The key to using magic in battle is resource and battlefield control. Mages are smart; a good mage always has a backup. Just like a good fighter always has a dagger in his boot or something hidden in his clothing in case he loses his sword or axe, a good mage will have a smaller focus or alternative focus item on his person in case his primary one is removed form his possession.
In the instances I just choose to say a smaller focus can only handle a certain number of uses, or maybe it can only use certain spells. But that's up to the roleplayers in question, so it's situational. Go with what works for the group, more or less. But don't sell your mage short. No caster is going to be dumb enough to not carry backup foci. Also, if we pay attention to the MSQ (slight spoiler here), Y'shtola casts without holding on to her focus (It's still on her belt), meaning you only technically need to have it on your person, but that's likely only if you're trained enough. I'm guessing that 'holding' the focus is a method that makes the casting easier, but when you're stronger, you just need it there in contact with you in one way or another. That's my headcanon anyway.