Terminus, the Warden of Eternity

Lore:
Nobody is sure exactly what Terminus is, or where it came from. The construct simply walked out of the ocean, apparently unharmed by the journey from lord knows where. Nobody knew what it was here for until it reached its destination: The League of Legends.
For the first time, Terminus displayed both the inclination and ability to communicate with humans. Terminus, a warden of time itself, came in response to the incident at Kalamanda. It demanded that the ones responsible for the massive time-distorting magic be brought forth so that it could dispense justice, and refused to leave until this was done. Unable to figure out what else they could do about it, the League bound Terminus to a summoning contract, under which it was trapped in an endless series of conflicts in the fields of battle.
Terminus does not seem to mind this turn of events. After all, bound as it may be, there is nothing in this world that can last forever.
Enemies: Zilean
NOTE: The above lore is out of date, now that Riot has retconned the League out of existence. I'm holding off on rewriting it until we know what happened to Kalamanda in the new canon.
Role: Tank, Mage
Health: 470 (+84)
Health Regen: 7.45 (+0.85)
Mana: 255 (+55)
Mana Regen: 6.25 (+0.5)
Range: 140 (melee)
Attack Damage: 50 (+3.9)
Attack Speed: 0.638 (+2.18%)
Armor: 24 (+3.6)
Magic Resist: 30 (+1.25)
Move Speed: 330
Passive: Inevitability
Every time you attack a champion, you set them as your target and gain one Inevitability stack. When you change targets or don't attack for a couple seconds, your stacks reset. Each Inevitability Stack gives you a +5% bonus to movement speed. Your current target is marked with a visual indicator, similar to Quinn's passive.
Q: Mark of Terminus
Target: One enemy champion
Cost: 60/70/80/90/100 mana
Range: 600
Magic Damage: 80/130/180/230/280 (+60% AP)
Cooldown: 13s/12s/11s/10s/9s
The target gains the Mark of Terminus for 2 seconds. While the Mark is active, you have vision of the target. If you press Q again while the Mark is active (following a 0.5-second buffer), Terminus moves in a flash, teleporting in a straight line towards the target and landing 200 units past their quarry's current position. The second activation terminates the Mark. Inflicts damage and begins cooldown when the Mark terminates - if the user didn't teleport, damage is reduced to 50%.
W: Resonance
Target: Circular AoE around self
Cost: 40 mana
Range: 350
Magic Damage: 75/100/125/150/175 (+50% AP)
Armor/MR Reduction: 2.5/3/3.5/4/4.5%
Cooldown: 10s
Passive: When you have a Target, they lose Armor/MR and you gain the same amount of Armor/MR, both effects multiplied by the number of Inevitability stacks you have.
Active: Consume all Inevitability Stacks to deal damage and slow all enemies in range by 40% for 1.5 seconds. For each stack consumed, damage is increased by 15%.
E: Eternity's Grip
Target: Line
Cost: 70/75/80/85/90 mana
Range: 800
Magic Damage: 60/110/160/210/260 (+60% AP)
Stun Duration: 1/1.25/1.5/1.75/2
Cooldown: 16/15/14/13/12
Fires a hook out from Terminus in a straight line, ignoring minions. If it hits a Champion, then it creates 1000-range tether binding the target to Terminus. As they walk further away, the link starts to turn red, and it snaps either after a couple seconds or if they move beyond tethering range. If it snaps due to distance, the enemy takes full damage and is stunned. If it snaps due to time, then the damage is halved, and the stun is replaced by a 50% slow.
If, due to moving away quickly after firing, the hook is already past 1000 units away from Terminus by the time it hits a champion, the tether length increases to 200 + the current distance.
R: Memento Mori
Target: Enemy Champion
Cost: 100/150/200 mana
Range: 1000
Blast Radius: 200
Magic Damage: 250/375/500 (+90% AP)
Cooldown: 150/135/120
Creates a powerful spectral missile that follows the target, bypassing terrain, structures and minions. The missile moves slowly enough that it can be easily outrun, but its duration is infinite, and it cannot be destroyed. If the champion enters their base, the missile will wait outside before continuing to chase. When it hits, it does heavy damage in a small radius - but it won't detonate for any target other than the one it's locked on to. It ignores minions, and if a champion other than its target gets in its way, the missile drops to 20%/35%/50% speed, pushing the champion that's blocking it. If it hits Yasuo's Wind Wall, then it is stopped, but not destroyed - it just presses against the wall until it disappears, then keeps moving.
As the missile flies, the damage it will inflict on a hit gradually increases, starting at 60% strength and reaching its maximum power after one minute of flight. Cooldown time begins once you fire, not once the missile hits. You can't have two missiles chasing the same champion, so the maximum missiles you can have in flight at once is five. If you upgrade the skill, then all in-flight missiles are instasntly upgraded.
Strategy Overview:
Terminus is a tank that focuses on locking on to specific enemies and refusing to let them escape. Mark of Terminus and Eternity's Grip can be used in conjunction to keep an enemy close and follow them should they try to get away. Mark of Terminus can function either as an engage to start a fight or as a way to pursue a fleeing enemy at the end of one. When playing against Terminus, being aware of the Mark is crucial - you have a brief window when you know that it could appear behind you at any moment, so that's your chance to try and lure it into a trap.
Resonance is a powerful tool, but the key is in the timing of activating it. You want to focus on a single enemy champion during team fights, in order to make the best use of your Inevitability stacks - then you want to hold off for as long as you can before popping them for the attack. After all, as long as the stacks are up, your target has reduced Armor which the rest of your team can exploit.
Memento Mori is a powerful attack, but its strength is not in its damage - as long as the missile is puttering around the map, it's putting pressure on the enemy team. When playing against Terminus, an important skill is identifying good times to just take the hit, rather than running away only to have the missile return when you're in more danger. Also, if a low-health ally has a missile heading their way, it can be good to physically block the missile with your body to give them time to escape.