~~Excessive Dota 2 copycat detected~~ I think this concept would really fit a champ in League.
_One of a lordly and magisterial race, Harbinger prowls the edge of the Void, sole surviving sentry of an outpost on the world at the rim of the abyss. From this jagged crystalline Outworld, forever on guard, he has gazed for eternities into the heavens, alert for any stirring in the bottomless night beyond the stars. Imprinted deep in the shining lattices of his intellect lies a resonant pattern akin to prophecy, a dark music implying that eventually some evil will wake out there, beyond the edges of creation, and turn its attention to our world. With his whole being focused on his vigil, Obsidian Destroyer paid little attention to events closer in to the sun. But at last the clamor of the Ancients, and a sense of growing threat from within as well as without, sent him winging sunward to visit the plains of war. Harbinger's place in our own prophecies is unambiguous: he must be considered an omen of worse things to come. But his arrival in itself is bad enough. _
Even the Dota one (this one) features the Void. That's a "GO!" sign for sure.
[P] Essence Aura
Casting spells in OD's vicinity has a chance (23%-40% chance based on level) to instantly restore 10% of maximum Mana. Can't be triggered by the same champ more often than every 2 seconds. On OD himself the effect is doubled to 20% and there is no 2 s inner cooldown.
All spells and autoattacks have 400 range.
[Q] Arcane Orb
Empowers the next attack with magic damage equal to 4%/5%/6%/7%/8%(+0.012%AP) of his current mana. Does not reset autoattack timer.
[W] Astral Imprisonment
_Type: ally/enemy champ targeted_
Briefly expels the target into his own dimension, hiding (like Zhonyas, but the target really disappears) the target for 0.8/1.1/1.4/1.7/2 seconds. If the target is an enemy, it loses 20/40/60/80/100 (+0.2 AP) of its maximum mana (affects the cap, not the current amount), and OD gains the same amount (also the cap) for 60 seconds. If the target is an ally, OD gives it 20/40/60/80/100 (+0.2 AP) of his own mana.
[E] Wings of Wisdom
_Type: enemy champ targeted_
Pushes OD and the target apart 200 units. For 4 autoattacks, OD can autoattack that target from 800 range with +40%/45%/50%/55%/60% attack speed. Both buffs are lost after 3 seconds or on target change.
[R] Sanity's Eclipse
_Type: AoE-targeted (AoE - 400)_
Overshadows all enemy champions in an area, dealing them magic damage based on a difference between his and target's maximum Mana: 1.4 (+0.0008 AP) for 1 mana difference, which is 1400 (+0.8 AP) for 1000 mana difference. For every champ hit OD gains 20/40/60/80/100 (+0.2 AP) maximum Mana (after damage is applied). If one of the targeted champions is affected by [E] Wings of Wisdom, [W] Astral Imprisonment is applied to him after [R] damage is applied. Previously [W] imprisoned enemy champions still take full damage despite their invulnerability. (If enemy's max mana is higher than OD's, he takes damage himself in the same ratio)
-very weak early game, Q deals tiny damage and costs incredibly much mana
-despite that, can escape ganks by [W]-ing himself or his attacker
-destructive late-game (on 6 slots can oneshot Vayne/Caitlyn/Kalista with a rightclick)
-can be played ADC by farming to a point when he has infinite mana from passive and hits ridiculously hard with [Q]
-can be played mid by maxing [W] and repeatedly using it on enemy midlaner, giving him mana issues and empowering [Q]
-he can be easily ran away from (kited easily, even though ranged)
-can briefly [W] hide his allies from harm, or enemies with trynda/alistar-style ults to reduce their powertime
-not _thicc_ - needs peel (a lot)
-high outplay potential
-no anti-tank tools when ult is down
-key stats: attack speed, max mana, cdr