Keystone Idea - Because you DEFINITELY don't have enough of these types of post.

Name: Overdrive (or something else idk)

Effect: You gain an active toggled ability (maybe in one of your summoner slots)

When activated, you rapidly lose mana per second The mana drained would probably be calculated as: (flat amount that increases with level) + (percentage of max mana) This way, you can better straddle the bar between building mana being useless (with just a max mana drain) and building mana being op (that's not something you get to say a lot ;p). Alternatively, you could have a purely max mana drain that enhances the stat increase provided by the ability if you have a higher mana pool.

The ability ends when your mana reaches zero. It cannot be cancelled once activated. So it's more of an instant/short cooldown ability than a "toggled" one. To avoid exploitation, you might have to give it a small cooldown (10-30 secs) and disable/reduce mana regen when the ability is active and/or on cooldown.

While active, your basic attacks deal bonus true damage based on your AP. Maybe an MS or defenses increase as well. My reasoning behind this is written below, but if anyone can think of a better buff for this spell to give then leave a comment by all means. Another cool mechanic would be a reset-style effect that gave a portion of your max mana back on kill or assist.

Good Users of this Keystone:

• High-cooldown mages, pop all your abilities then use your extra mana to stay relevant in a fight.
• Hybrid attackers, Kog'maw Ezreal etc.*
• Literally anyone who builds nashor's tooth, wit's end, rageblade or even lich bane
• Healing oriented champs who have AP scaling, but not enough damage in their kits

*not Corki because his passive converts AD to magic on-hit damage, not AP

Thoughts and Reasons:

I'll begin by admitting that this idea probably would have more impact on the game than any one keystone rune should. I nevertheless think it would be a fun inclusion for the following reasons:

1. it validates a number of underused playstyles, eg. on-hit AP and manamune builds in general
2. it encourages versatile and creative itemization
3. it adds another layer of strategy onto a fight

There are still a number of issues that would need to be addressed in implementing this idea: Its all-or-nothing gameplay elements could be inherantly unhealthy in many situations (think drain-tanks). A typical combat scenario with this keystone could result in many outcomes, some of which are quite extreme...

1. you use overdrive to stop a backline dive or to secure a pick. You got a kill, but you can't really participate in the oncoming teamfight (healthy)
2. you string together multiple kills using your reset mechanic (seems to work in the current game with certain champs)
3. you pop overdrive and everyone kites away from you until it expires (hard to prevent in solo queue without coordinated CC). This could cause you to lose your last crucial teamfight in a game.
4. you make a decision to not use the ability - you essentially have no keystone and one less summoner spell (your withholding it does provide some pressure however).

It would be hard to ensure that this ability is balanced fairly for a wide pool of champs. TF is an example of an champ that could exploit this rune at the cost of it being nerfed too hard to be used by others. His on-hit passive ability allows him to still be useful with no mana. He could become too good at getting picks across the map; you would have no opportunity for counterplay once he ults/gold cards then pops overdrive. The reset mechanic also would make it WAY too easy to counter-gank bot lane with ult and get a triple kill. AP Kog could become a ridiculous tank-shredder with his max-health AP scaling on his w combined with extra true damage (the true damage aspect helps with accessibility to hybrids and enchanters who can't build so much armour/magic pen btw). AP Kog is probably easy enough to shut down early :p. While we're on the topic, the amount of troll builds that this ability would encourage is also a definite risk. Teemo (another on-hit AP passive), Azir and Lulu may also have to be nerfed for the glory of battle-soraka and a variety of other "strategies". It would also have to be completely inaccessible to non-mana-users.

Lastly, this ability actively encourages you, in a fair few cases, to use your abilities sparingly and just mindlessly auto-attack the enemy team. This, combined with the at-times-frustrating risk-reward nature of the ability, could cause this rune to be something that's fun to think about pre-game but feels really awful and same-y to play. I'm also not sure if this is a good fit for the inspiration tree.

Thank you to anyone who takes the time to read this. It was fun to write, being my first post of this kind. I don't claim to be an expert at this game. These are just my thoughts and I'd love to hear from anyone else who wants to add to this discussion.

Cheers, Chrizazar