This concept is pretty old and it's been on my PC for a long time. I never published it because its lore felt not good enough - I changed it milions of times, trying to tie this champion - or currently just kit - either to my long-forgotten Project: Fyrone, or to Hirana Monastery. In order to be able to think clearly, I need to get this kit off my head and the only way to do so is to publish it. With or without a lore and personality. Pure kit.
While this concept started as a rework for Old Karma, I quickly replaced almost all and then all of her abilities. Then she lost her personality and somehow - I scrapped the rework and started a new champion, whose first and perfect, I believe, use is to punish enemy casters - even those who build AD. Nahtan - because this was supposed to be his name - is an assassin or maybe fighter or maybe even something else (I'm so confused) meant to survive enemy casters' combos and then get rid of them, preferably not far from main teamfight (Gold and worse players will understand this better than any other...).
His skills are also quite good in laning phase and can set up ganks for his jungler, and let him survive and disengage from enemy ganks if used properly. RIP interpunction.
Nahtan, Custodian of Hiranian Library, uses something between "Old Mantra kit archetype" and "New Mantra kit archetype". His ultimate is automatically learned at level 1, and it can be upgraded at levels 6, 11 and 16. It is used to empower his basic abilities.
Keeper of the secrets
Nearby enemies are marked with Punishment Deserved stack for every spell they cast for 4 seconds, up to 5 times. Nahtan's basic attacks, Spirit Slash and Spiritual Swing deal bonus 5 / 8 / 11 / 14 / 17(+ 5% AP) magic damage for every stack on target. Silenced foes take maximum damage, no matter how many stacks they are marked with.
Keeper of the secrets consists of two components. First passively marks nearby enemies, while second, also passively, deals bonus magic damage to targets marked with first component of Keeper of the secrets when they are damaged with Nahtan's basic attacks, Spirit Slash or Spiritual Swing.
Nahtan mimicks Spirit Slash in a full circle, dealing physical damage to enemies hit.
Spiritual Swing also leaves an area of effect for three seconds, which causes enemies to take magic damage upon spellcast and silences them afterwards.
On-spell damage: 80 / 150 / 220 / 290 (+ 50% AP)
Silence Duration: 1 / 1.25 / 1.5 / 1.75
Spirit Slash is a conic area of effect skill shot that deals damage from left to right during its brief cast time. Its empowered version, Spiritual Swing is an point blank area of effect ability that similarly deals damage from left to right during the same brief cast time, but in a full circle around him.
Nahtan then readies to parry first enemy champion's basic attack coming from 90' cone in front of him within one second.
Nahtan mimicks Fool-proof with an increased range. Enemy champions Nahtan passes are disarmed in addition to being slowed.
Disarm Duration: 1 / 1.25 / 1.5 / 1.75
Nahtan then readies to parry any incoming basic attacks for one second.
Fool-proof and Untouchable are a sort of ground targeted, pass-through dash, with an autocast self-buff component that parries next incoming champion basic attack from a cone in front of Nahtan in Fool-proof's case, or all incoming champion basic attacks for the next second in Untouchable's case.
Spell shields will block slow and disarm effect, but parry effect won't be denied, as it's a self-buff.
If no enemy champions are passed through, Nahtan faces his previous direction.
COST: 100 / 90 / 80 / 70 / 60 mana
COOLDOWN: 14 / 13 / 12 / 11 / 10
Nahtan binds to first enemy champion hit, stealing their movement speed over two seconds. At the end of duration or when the links is broken, target takes magic damage proportional to distance travelled. This damage has a cap scaling with ranks of Spirit Knot.
Movement speed modificators then decay over one second, but can be refreshed by dashing through that target with Fool-proof or Untouchable.
Movement speed stolen: 15 / 20 / 25 / 30 / 35% (+ 2.5% per 100AP)
Magic damage per unit travelled: 0.2 / 0.3 / 0.4 / 0.5 / 0.6 ( +0.5% AP)
Damage cap: 70 / 120 / 170 / 230 / 280 (+ 40% AP)
Nahtan additionally receives attack speed bonus equal to movement speed stolen. Damage proportional to distance travelled is also applied upon every target's spellcast.
Spirit Knot and Fetters are linear, colliding skill shots that steal movement speed from enemy champion hit over two seconds and deal single-target magic damage at the end of duration or when enemy struck casts a spell in Spirit Fetters' case. Spirit Fetters additionally grants Nahtan bonus attack speed equal to mevement speed stolen.
Nahtan empowers his next ability cast within the next 8 seconds, adding an additional effect. Empowered abilities share the same cooldown as their basic form. Nahtan can have up to two stacks of Mantra at once. Each spellcast consumes only one stack.
Mantra is a stackable self-buff ability.
Mantras can be queued up, causing two next abilities to be empowered.
Mantra has no cast time.
Edit: Thanks for suggestions and errors pointed out in the comments. I updated the post with them in mind.