- Karthus before his Visual Upgrade was one of five champions voiced by Adam Harrington, the other four being , , , and .
- Karthus' ultimate, Requiem, has the same name as the Catholic gathering called the "Requiem Mass", which is dedicated to praying for the souls of the deceased.
- The old icon for Karthus' passive, Death Defied was a sheep.
- Karthus is one of the four champions to have abilities which allow them to continue to attack after death. Karthus's ability being Death Defied and the others being Icathian Surprise , Omen of Death and Rise of the Thorns .
- There is a glass case which has "Press R to Win" in all caps in the video of the Mac Version Launch which possibly references people playing as Karthus stating Press R to Win.
- Анимация смерти Картуса возможно является отсылкой к мультфильму "Труп Невесты" Тима Бёртона, где главная героиня также улетела бабочками в небо.
- Has special quotes for , , , , , , , and .
- Has special quotes for Items like and .
- Karthus after the VU has the most champion quotes to date, numbering at 136 (Including the ones for his Pentakill skin).
- "I am become death" is a reference to Robert Oppenheimer, a physicist who created the Atomic bomb 
- His joke "What's the incantation again... yes, erm... oh, right, R!" is a community meme joke where Karthus' ultimate is stated as "Press R to win"
- His joke "I'm putting your name in my little black necronomicon." is a possible, but unlikely, reference to the Death Note series, while it also relates to the works of H.P. Lovecraft.
- His quote "You can't kill the metal!" is a reference to the song "The Metal" by Tenacious D.
- His quotes "Silence, groundwalker.", "What have I done? Created beauty by simply rocking! That's all.", and "What is that sound? It's the pounding of creation's hammer on the anvil of time!" are references to the video game "Brütal Legend".
- His quote "Towerdiver, gotta get away, get away" is a reference to the song "Holy Diver" by Dio.
- His quote "Die for metal" is a reference to the song "Die for Metal" by Manowar.
- His quote "Light's out forever" is a reference to the song "School's Out" by Alice Cooper.
- His quote "Death is blacker than the blackest black... times infinity." is a reference to the popular Adult Swim show, Metalocalypse.
- His quote "This is eleven!" is a reference to the band Spinal Tap.
- His quote "I am the lightbringer!" is a reference to Pentakill's song "Lightbringer". It is part of the band's newest album, "Smite and Ignite".
- Both the quotes "Everyone will falter when you die!" and "My song, smite them all!" (modification of "Son, smite them all!") are from Pentakill's song "Thornmail" of the album "Smite and Ignite."
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- In Karthus's Chinese splash artworks, he does not have a skull face (except for Grim Reaper and Pentakill). This is likely because the use of exposed bones in video-games is explicitly forbidden in China (a famous example is the race Undead in .
, where in the American server their bones are exposed, while in the Chinese server they're covered)
- Riot has since addressed the issue with Karthus's VU, taking away his skeletal features.
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- The reworked version of this skin resembles the previous Chinese splash art for Phantom Karthus.
- Phantom Karthus is most likely a reference to the Phantom of the Opera, due to the mask on the left side of his face.
- Phantom Karthus VU is most likely a reference to Sauron from the Lord of the Rings, while is staff resembles the Eye of Sauron.
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- The Statue of Karthus is a reference to the in .
- When he dances, you can hear a eagle shout as well.
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- He has the scythe that
is usually depicted to be wielding rather than his usual staff.
- The book he is holding in Grim Reaper skin it is known as the "Necronomicon" a fictional book of the dead, attributed to the works of H.P. Lovecraft.
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- Following his VU, Pentakill Karthus has special voiceover lines.
- This skin is inspired by the British heavy metal group maskot from .
- Karthus originally was a self-appointed Over-lord of Howling Marsh before his lore changed into being working for Shadow Isles.
- Karthus is the fourth member of the famous Pentakill (Band) with , , and . Karthus takes the role of Lead Singer as Pentakill Karthus, "the Deathsinger".
- They have since released an album titled "Smite and Ignite" which was released on June 3, 2014 for free.
- Karthus was present in a fight between Fields of Justice - The Journal of Justice: Volume 1, Issue 21. and . He was found "dead" at the scene, but he was revived since then. He stated that he was "protecting the inhibitor," referring to something other than the inhibitors on the