Keeper of the Light/Counters

Bad against...

 * Night Stalker can reduce most of the KotL's power turning day into night.


 * Nyx Assassin is a full-fledge ganker, Keeper's low strength gain and health pool can be suspectible to Nyx's Vendetta.
 * Keeper has balanced mana pool and intelligence, giving Nyx to cast Mana Burn.


 * Attacking Abaddon through out the early game when he's level 6, Abaddon can soak up damage through Aphotic Shield and Borrowed Time.


 * Keeper of the Light is easy target victory for Legion Commander to Duel.


 * Sven is common Black King Bar carrier and can easily kill Kotl with God's Strength during Storm Hammer's stun.


 * Mana leak is almost useless against Antimage, while he is very dangerous for Kotl.


 * Mana leak is useless against Storm's jumps because of his invulnerability.

Others

 * Keeper of the Light is very susceptible to all gankers in all stages of the game due to his frailness and lack of any escape mechanism.
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Items

 * Black King Bar ignores all magical damage and target point skill.
 * Lotus Orb can be a serious item counter to Keeper of the Light's Mana Leak, severely hindering him and draining his mana pool as well.
 * Blade Mail can severely damage Keeper of the Light because of his low health pool.

Good against...

 * Keeper of the Light can easily drain mana from Timbersaw, meanwhile mana is very necessary for Timbersaw's spells and overall utility.


 * For the same reason as Timbersaw, the already low mana pool Earth Spirit needs a lot of mana for his spells.


 * Both Mana Leak and Blinding Light from Keeper of the Light will be a problem for Bristleback. Mana Leak because Bristle grows stronger if he uses first two abilities, and Blinding Light is because Bristleback, like many other heroes, should chase enemies.


 * Keeper of the Light's Mana Leak / Blinding Light combo will be a problem for Wraith King because Wraith King has low mana pool and mana regeneration, which his ultimate Reincarnation requires a high amount of it.


 * Keeper of the Light's exceptionally long cast range for Mana Leak can effectively disable Monkey King's flexible motility and low mana pool. Also, Blinding Light can knock Monkey King outside of his ultimate, Wukong's Command or can generally blind him effectively. Aghanim Scepter also gives him unobstructed vision during daytime, which is very important against Monkey King.

Others

 * Every Hero that is mana-reliant, especially those with low mana pool and high mana cost spells (ex. Earthshaker, Pudge, Storm Spirit and etc.) can easily be disabled with a Mana Leak / Blinding Light combo, making clashes easier to win.
 * Mobile heroes, such as Weaver, Monkey King and etc., are easily disabled using Mana Leak, making ganking or chasing more successful.
 * Illuminate's high magic damage is effective against unsuspecting low movement speed, low health pool and low magic resistance heroes such as Lion, Shadow Shaman, Shadow Demon and etc., especially during the early laning phase.

Works well with...

 * Anti-Mage's Mana Void works in synergy with the Mana Leak's drain on enemy mana pool, allowing maximum spell damage.


 * Faceless Void's Chronosphere allows Keeper Of The Light to deal maximum damage from its ability, Illuminate


 * Shaman's ability, Shackles, Keeper Of The Light allows to deal maximum damage from its ability, Illuminate.


 * Clockwerk's ability, Power Cogs, allows Keeper to deal maximum damage from its ability, Illuminate.


 * Mana Leak limits enemy movement, making easier hooks. It can also effectively drain mana during the hook.

Others

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List of heroes that counter Keeper of the Light, and list of heroes that work well with Keeper of the Light.